Network Working Group J. Moy
Request for Comments: 1247 Proteon, Inc.
Obsoletes: RFC 1131 July 1991
OSPF Version 2
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the ``IAB Official Protocol
Standards'' for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
Abstract
This memo documents version 2 of the OSPF protocol. OSPF is a link-
state based routing protocol. It is designed to be run internal to a
single Autonomous System. Each OSPF router maintains an identical
database describing the Autonomous System's topology. From this
database, a routing table is calculated by constructing a shortest-path
tree.
OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic. OSPF provides support
for equal-cost multipath. Separate routes can be calculated for each IP
type of service. An area routing capability is provided, enabling an
additional level of routing protection and a reduction in routing
protocol traffic. In addition, all OSPF routing protocol exchanges are
authenticated.
Version 1 of the OSPF protocol was documented in RFC 1131. The
differences between the two versions are explained in Appendix F.
Please send comments to ospf@trantor.umd.edu.
1. Introduction
This document is a specification of the Open Shortest Path First (OSPF)
internet routing protocol. OSPF is classified as an Internal Gateway
Protocol (IGP). This means that it distributes routing information
between routers belonging to a single Autonomous System. The OSPF
protocol is based on SPF or link-state technology. This is a departure
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from the Bellman-Ford base used by traditional internet routing
protocols.
The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force. It has been designed expressly for the
internet environment, including explicit support for IP subnetting,
TOS-based routing and the tagging of externally-derived routing
information. OSPF also provides for the authentication of routing
updates, and utilizes IP multicast when sending/receiving the updates.
In addition, much work has been done to produce a protocol that responds
quickly to topology changes, yet involves small amounts of routing
protocol traffic.
The author would like to thank Rob Coltun, Milo Medin, Mike Petry and
the rest of the OSPF working group for the ideas and support they have
given to this project.
1.1 Protocol overview
OSPF routes IP packets based solely on the destination IP address and IP
Type of Service found in the IP packet header. IP packets are routed
"as is" -- they are not encapsulated in any further protocol headers as
they transit the Autonomous System. OSPF is a dynamic routing protocol.
It quickly detects topological changes in the AS (such as router
interface failures) and calculates new loop-free routes after a period
of convergence. This period of convergence is short and involves a
minimum of routing traffic.
In an SPF-based routing protocol, each router maintains a database
describing the Autonomous System's topology. Each participating router
has an identical database. Each individual piece of this database is a
particular router's local state (e.g., the router's usable interfaces
and reachable neighbors). The router distributes its local state
throughout the Autonomous System by flooding.
All routers run the exact same algorithm, in parallel. From the
topological database, each router constructs a tree of shortest paths
with itself as root. This shortest-path tree gives the route to each
destination in the Autonomous System. Externally derived routing
information appears on the tree as leaves.
OSPF calculates separate routes for each Type of Service (TOS). When
several equal-cost routes to a destination exist, traffic is distributed
equally among them. The cost of a route is described by a single
dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a grouping is
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called an area. The topology of an area is hidden from the rest of the
Autonomous System. This information hiding enables a significant
reduction in routing traffic. Also, routing within the area is
determined only by the area's own topology, lending the area protection
from bad routing data. An area is a generalization of an IP subnetted
network.
OSPF enables the flexible configuration of IP subnets. Each route
distributed by OSPF has a destination and mask. Two different subnets
of the same IP network number may have different sizes (i.e., different
masks). This is commonly referred to as variable length subnets. A
packet is routed to the best (i.e., longest or most specific) match.
Host routes are considered to be subnets whose masks are "all ones"
(0xffffffff).
All OSPF protocol exchanges are authenticated. This means that only
trusted routers can participate in the Autonomous System's routing. A
variety of authentication schemes can be used; a single authentication
scheme is configured for each area. This enables some areas to use much
stricter authentication than others.
Externally derived routing data (e.g., routes learned from the Exterior
Gateway Protocol (EGP)) is passed transparently throughout the
Autonomous System. This externally derived data is kept separate from
the OSPF protocol's link state data. Each external route can also be
tagged by the advertising router, enabling the passing of additional
information between routers on the boundaries of the Autonomous System.
1.2 Definitions of commonly used terms
Here is a collection of definitions for terms that have a specific
meaning to the protocol and that are used throughout the text. The
reader unfamiliar with the Internet Protocol Suite is referred to [RS-
85-153] for an introduction to IP.
Router
A level three Internet Protocol packet switch. Formerly called a
gateway in much of the IP literature.
Autonomous System
A group of routers exchanging routing information via a common
routing protocol. Abbreviated as AS.
Internal Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous System has
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a single IGP. Different Autonomous Systems may be running different
IGPs.
Router ID
A 32-bit number assigned to each router running the OSPF protocol.
This number uniquely identifies the router within an Autonomous
System.
Network
In this paper, an IP network or subnet. It is possible for one
physical network to be assigned multiple IP network/subnet numbers.
We consider these to be separate networks. Point-to-point physical
networks are an exception - they are considered a single network no
matter how many (if any at all) IP network/subnet numbers are
assigned to them.
Network mask
A 32-bit number indicating the range of IP addresses residing on a
single IP network/subnet. This specification displays network masks
as hexadecimal numbers. For example, the network mask for a class C
IP network is displayed as 0xffffff00. Such a mask is often
displayed elsewhere in the literature as 255.255.255.0.
Multi-access networks
Those physical networks that support the attachment of multiple
(more than two) routers. Each pair of routers on such a network is
assumed to be able to communicate directly (e.g., multi-drop
networks are excluded).
Interface
The connection between a router and one of its attached networks.
An interface has state information associated with it, which is
obtained from the underlying lower level protocols and the routing
protocol itself. An interface to a network has associated with it a
single IP address and mask (unless the network is an unnumbered
point-to-point network). An interface is sometimes also referred to
as a link.
Neighboring routers
Two routers that have interfaces to a common network. On multi-
access networks, neighbors are dynamically discovered by OSPF's
Hello Protocol.
Adjacency
A relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of
neighboring routers become adjacent.
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Link state advertisement
Describes to the local state of a router or network. This includes
the state of the router's interfaces and adjacencies. Each link
state advertisement is flooded throughout the routing domain. The
collected link state advertisements of all routers and networks
forms the protocol's topological database.
Hello protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On multi-access networks the Hello protocol
can also dynamically discover neighboring routers.
Designated Router
Each multi-access network that has at least two attached routers has
a Designated Router. The Designated Router generates a link state
advertisement for the multi-access network and has other special
responsibilities in the running of the protocol. The Designated
Router is elected by the Hello Protocol.
The Designated Router concept enables a reduction in the number of
adjacencies required on a multi-access network. This in turn
reduces the amount of routing protocol traffic and the size of the
topological database.
Lower-level protocols
The underlying network access protocols that provide services to the
Internet Protocol and in turn the OSPF protocol. Examples of these
are the X.25 packet and frame levels for PDNs, and the ethernet data
link layer for ethernets.
1.3 Brief history of SPF-based routing technology
OSPF is an SPF-based routing protocol. Such protocols are also referred
to in the literature as link-state or distributed-database protocols.
This section gives a brief description of the developments in SPF-based
technology that have influenced the OSPF protocol.
The first SPF-based routing protocol was developed for use in the
ARPANET packet switching network. This protocol is described in
[McQuillan]. It has formed the starting point for all other SPF-based
protocols. The homogeneous Arpanet environment, i.e., single-vendor
packet switches connected by synchronous serial lines, simplified the
design and implementation of the original protocol.
Modifications to this protocol were proposed in [Perlman]. These
modifications dealt with increasing the fault tolerance of the routing
protocol through, among other things, adding a checksum to the link
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state advertisements (thereby detecting database corruption). The paper
also included means for reducing the routing traffic overhead in an
SPF-based protocol. This was accomplished by introducing mechanisms
which enabled the interval between link state advertisements to be
increased by an order of magnitude.
An SPF-based algorithm has also been proposed for use as an ISO IS-IS
routing protocol. This protocol is described in [DEC]. The protocol
includes methods for data and routing traffic reduction when operating
over broadcast networks. This is accomplished by election of a
Designated Router for each broadcast network, which then originates a
link state advertisement for the network.
The OSPF subcommittee of the IETF has extended this work in developing
the OSPF protocol. The Designated Router concept has been greatly
enhanced to further reduce the amount of routing traffic required.
Multicast capabilities are utilized for additional routing bandwidth
reduction. An area routing scheme has been developed enabling
information hiding/protection/reduction. Finally, the algorithm has
been modified for efficient operation in the internet environment.
1.4 Organization of this document
The first three sections of this specification give a general overview
of the protocol's capabilities and functions. Sections 4-16 explain the
protocol's mechanisms in detail. Packet formats, protocol constants,
configuration items and required management statistics are specified in
the appendices.
Labels such as HelloInterval encountered in the text refer to protocol
constants. They may or may not be configurable. The architectural
constants are explained in Appendix B. The configurable constants are
explained in Appendix C.
The detailed specification of the protocol is presented in terms of data
structures. This is done in order to make the explanation more precise.
Implementations of the protocol are required to support the
functionality described, but need not use the precise data structures
that appear in this paper.
2. The Topological Database
The database of the Autonomous System's topology describes a directed
graph. The vertices of the graph consist of routers and networks. A
graph edge connects two routers when they are attached via a physical
point-to-point network. An edge connecting a router to a network
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indicates that the router has an interface on the network.
The vertices of the graph can be further typed according to function.
Only some of these types carry transit data traffic; that is, traffic
that is neither locally originated nor locally destined. Vertices that
can carry transit traffic are indicated on the graph by having both
incoming and outgoing edges.
Vertex type Vertex name Transit?
_____________________________________
1 Router yes
2 Network yes
3 Stub network no
Table 1: OSPF vertex types.
OSPF supports the following types of physical networks:
Point-to-point networks
A network that joins a single pair of routers. A 56Kb serial line
is an example of a point-to-point network.
Broadcast networks
Networks supporting many (more than two) attached routers, together
with the capability to address a single physical message to all of
the attached routers (broadcast). Neighboring routers are
discovered dynamically on these nets using OSPF's Hello Protocol.
The Hello Protocol itself takes advantage of the broadcast
capability. The protocol makes further use of multicast
capabilities, if they exist. An ethernet is an example of a
broadcast network.
Non-broadcast networks
Networks supporting many (more than two) routers, but having no
broadcast capability. Neighboring routers are also discovered on
these nets using OSPF's Hello Protocol. However, due to the lack of
broadcast capability, some configuration information is necessary
for the correct operation of the Hello Protocol. On these networks,
OSPF protocol packets that are normally multicast need to be sent to
each neighboring router, in turn. An X.25 Public Data Network (PDN)
is an example of a non-broadcast network.
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The neighborhood of each network node in the graph depends on whether
the network has multi-access capabilities (either broadcast or non-
broadcast) and, if so, the number of routers having an interface to the
network. The three cases are depicted in Figure 1. Rectangles indicate
routers. Circles and oblongs indicate multi-access networks. Router
names are prefixed with the letters RT and network names with N. Router
interface names are prefixed by I. Lines between routers indicate
point-to-point networks. The left side of the figure shows a network
with its connected routers, with the resulting graph shown on the right.
Two routers joined by a point-to-point network are represented in the
directed graph as being directly connected by a pair of edges, one in
each direction. Interfaces to physical point-to-point networks need not
be assigned IP addresses. Such a point-to-point network is called
unnumbered. The graphical representation of point-to-point networks is
designed so that unnumbered networks can be supported naturally. When
interface addresses exist, they are modelled as stub routes. Note that
each router would then have a stub connection to the other router's
interface address (see Figure 1).
When multiple routers are attached to a multi-access network, the
directed graph shows all routers bidirectionally connected to the
network vertex (again, see Figure 1). If only a single router is
attached to a multi-access network, the network will appear in the
directed graph as a stub connection.
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes on the
network. Hosts attached directly to routers (referred to as host
routes) appear on the graph as stub networks. The network mask for a
host route is always 0xffffffff, which indicates the presence of a
single node.
Figure 2 shows a sample map of an Autonomous System. The rectangle
labelled H1 indicates a host, which has a SLIP connection to router
RT12. Router RT12 is therefore advertising a host route. Lines between
______________________________________
(Figure not included in text version.)
Figure 1: Network map components
______________________________________
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routers indicate physical point-to-point networks. The only point-to-
point network that has been assigned interface addresses is the one
joining routers RT6 and RT10. Routers RT5 and RT7 have EGP connections
to other Autonomous Systems. A set of EGP-learned routes have been
displayed for both of these routers.
A cost is associated with the output side of each router interface.
This cost is configurable by the system administrator. The lower the
cost, the more likely the interface is to be used to forward data
traffic. Costs are also associated with the externally derived routing
data (e.g., the EGP-learned routes).
The directed graph resulting from the map in Figure 2 is depicted in
Figure 3. Arcs are labelled with the cost of the corresponding router
output interface. Arcs having no labelled cost have a cost of 0. Note
that arcs leading from networks to routers always have cost 0; they are
significant nonetheless. Note also that the externally derived routing
data appears on the graph as stubs.
The topological database (or what has been referred to above as the
directed graph) is pieced together from link state advertisements
generated by the routers. The neighborhood of each transit vertex is
represented in a single, separate link state advertisement. Figure 4
shows graphically the link state representation of the two kinds of
transit vertices: routers and multi-access networks. Router RT12 has an
______________________________________
(Figure not included in text version.)
Figure 2: A sample Autonomous System
______________________________________
__________________________________________
(Figures not included in text version.)
Figure 3: The resulting directed graph
Figure 4: Individual link state components
__________________________________________
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interface to two broadcast networks and a SLIP line to a host. Network
N6 is a broadcast network with three attached routers. The cost of all
links from network N6 to its attached routers is 0. Note that the link
state advertisement for network N6 is actually generated by one of the
attached routers: the router that has been elected Designated Router for
the network.
2.1 The shortest-path tree
When no OSPF areas are configured, each router in the Autonomous System
has an identical topological database, leading to an identical graphical
representation. A router generates its routing table from this graph by
calculating a tree of shortest paths with the router itself as root.
Obviously, the shortest-path tree depends on the router doing the
calculation. The shortest-path tree for router RT6 in our example is
depicted in Figure 5.
The tree gives the entire route to any destination network or host.
However, only the next hop to the destination is used in the forwarding
process. Note also that the best route to any router has also been
calculated. For the processing of external data, we note the next hop
and distance to any router advertising external routes. The resulting
routing table for router RT6 is pictured in Table 2. Note that there is
a separate route for each end of a numbered serial line (in this case,
the serial line between routers RT6 and RT10).
Routes to networks belonging to other AS'es (such as N12) appear as
dashed lines on the shortest path tree in Figure 5. Use of this
externally derived routing information is considered in the next
section.
______________________________________
(Figure not included in text version.)
Figure 5: The SPF tree for router RT6
______________________________________
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Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8
Table 2: The portion of router RT6's routing table listing local
destinations.
2.2 Use of external routing information
After the tree is created the external routing information is examined.
This external routing information may originate from another routing
protocol such as EGP, or be statically configured (static routes).
Default routes can also be included as part of the Autonomous System's
external routing information.
External routing information is flooded unaltered throughout the AS. In
our example, all the routers in the Autonomous System know that router
RT7 has two external routes, with metrics 2 and 9.
OSPF supports two types of external metrics. Type 1 external metrics
are equivalent to the link state metric. Type 2 external metrics are
greater than the cost of any path internal to the AS. Use of Type 2
external metrics assumes that routing between AS'es is the major cost of
routing a packet, and eliminates the need for conversion of external
costs to internal link state metrics.
Here is an example of Type 1 external metric processing. Suppose that
the routers RT7 and RT5 in Figure 2 are advertising Type 1 external
metrics. For each external route, the distance from Router RT6 is
calculated as the sum of the external route's cost and the distance from
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Router RT6 to the advertising router. For every external destination,
the router advertising the shortest route is discovered, and the next
hop to the advertising router becomes the next hop to the destination.
Both Router RT5 and RT7 are advertising an external route to destination
network N12. Router RT7 is preferred since it is advertising N12 at a
distance of 10 (8+2) to Router RT6, which is better than router RT5's 14
(6+8). Table 3 shows the entries that are added to the routing table
when external routes are examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of router RT6's routing table listing external
destinations.
Processing of Type 2 external metrics is simpler. The AS boundary
router advertising the smallest external metric is chosen, regardless of
the internal distance to the AS boundary router. Suppose in our example
both router RT5 and router RT7 were advertising Type 2 external routes.
Then all traffic destined for network N12 would be forwarded to router
RT7, since 2 < 8. When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used to break the tie.
Both Type 1 and Type 2 external metrics can be present in the AS at the
same time. In that event, Type 1 external metrics always take
precedence.
This section has assumed that packets destined for external destinations
are always routed through the advertising AS boundary router. This is
not always desirable. For example, suppose in Figure 2 there is an
additional router attached to network N6, called Router RTX. Suppose
further that RTX does not participate in OSPF routing, but does exchange
EGP information with the AS boundary router RT7. Then, router RT7 would
end up advertising OSPF external routes for all destinations that should
be routed to RTX. An extra hop will sometimes be introduced if packets
for these destinations need always be routed first to router RT7 (the
advertising router).
To deal with this situation, the OSPF protocol allows an AS boundary
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router to specify a "forwarding address" in its external advertisements.
In the above example, Router RT7 would specify RTX's IP address as the
"forwarding address" for all those destinations whose packets should be
routed directly to RTX.
The "forwarding address" has one other application. It enables routers
in the Autonomous System's interior to function as "route servers". For
example, in Figure 2 the router RT6 could become a route server, gaining
external routing information through a combination of static
configuration and external routing protocols. RT6 would then start
advertising itself as an AS boundary router, and would originate a
collection of OSPF external advertisements. In each external
advertisement, router RT6 would specify the correct Autonomous System
exit point to use for the destination through appropriate setting of the
advertisement's "forwarding address" field.
2.3 Equal-cost multipath
The above discussion has been simplified by considering only a single
route to any destination. In reality, if multiple equal-cost routes to
a destination exist, they are all discovered and used. This requires no
conceptual changes to the algorithm, and its discussion is postponed
until we consider the tree-building process in more detail.
With equal cost multipath, a router potentially has several available
next hops towards any given destination.
2.4 TOS-based routing
OSPF can calculate a separate set of routes for each IP Type of Service.
The IP TOS values are represented in OSPF exactly as they appear in the
IP packet header. This means that, for any destination, there can
potentially be multiple routing table entries, one for each IP TOS.
Up to this point, all examples shown have assumed that routes do not
vary on TOS. In order to differentiate routes based on TOS, separate
interface costs can be configured for each TOS. For example, in Figure
2 there could be multiple costs (one for each TOS) listed for each
interface. A cost for TOS 0 must always be specified.
When interface costs vary based on TOS, a separate shortest path tree is
calculated for each TOS (see Section 2.1). In addition, external costs
can vary based on TOS. For example, in Figure 2 router RT7 could
advertise a separate type 1 external metric for each TOS. Then, when
calculating the TOS X distance to network N15 the cost of the shortest
TOS X path to RT7 would be added to the TOS X cost advertised by RT7
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(see Section 2.2).
All OSPF implementations must be capable of calculating routes based on
TOS. However, OSPF routers can be configured to route all packets on
the TOS 0 path (see Appendix C), eliminating the need to calculate non-
zero TOS paths. This can be used to conserve routing table space and
processing resources in the router. These TOS-0-only routers can be
mixed with routers that do route based on TOS. TOS-0-only routers will
be avoided as much as possible when forwarding traffic requesting a
non-zero TOS.
It may be the case that no path exists for some non-zero TOS, even if
the router is calculating non-zero TOS paths. In that case, packets
requesting that non-zero TOS are routed along the TOS 0 path (see
Section 11.1).
3. Splitting the AS into Areas
OSPF allows collections of contiguous networks and hosts to be grouped
together. Such a group, together with the routers having interfaces to
any one of the included networks, is called an area. Each area runs a
separate copy of the basic SPF routing algorithm. This means that each
area has its own topological database and corresponding graph, as
explained in the previous section.
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic as
compared to treating the entire Autonomous System as a single SPF
domain.
With the introduction of areas, it is no longer true that all routers in
the AS have an identical topological database. A router actually has a
separate topological database for each area it is connected to.
(Routers connected to multiple areas are called area border routers).
Two routers belonging to the same area have, for that area, identical
area topological databases.
Routing in the Autonomous System takes place on two levels, depending on
whether the source and destination of a packet reside in the same area
(intra-area routing is used) or different areas (inter-area routing is
used). In intra-area routing, the packet is routed solely on
information obtained within the area; no routing information obtained
from outside the area can be used. This protects intra-area routing
from the injection of bad routing information. We discuss inter-area
routing in Section 3.2.
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RFC 1247 OSPF Version 2 July 1991
3.1 The backbone of the Autonomous System
The backbone consists of those networks not contained in any area, their
attached routers, and those routers that belong to multiple areas. The
backbone must be contiguous.
It is possible to define areas in such a way that the backbone is no
longer contiguous. In this case the system administrator must restore
backbone connectivity by configuring virtual links.
Virtual links can be configured between any two backbone routers that
have an interface to a common non-backbone area. Virtual links belong
to the backbone. The protocol treats two routers joined by a virtual
link as if they were connected by an unnumbered point-to-point network.
On the graph of the backbone, two such routers are joined by arcs whose
costs are the intra-area distances between the two routers. The routing
protocol traffic that flows along the virtual link uses intra-area
routing only.
The backbone is responsible for distributing routing information between
areas. The backbone itself has all of the properties of an area. The
topology of the backbone is invisible to each of the areas, while the
backbone itself knows nothing of the topology of the areas.
3.2 Inter-area routing
When routing a packet between two areas the backbone is used. The path
that the packet will travel can be broken up into three contiguous
pieces: an intra-area path from the source to an area border router, a
backbone path between the source and destination areas, and then another
intra-area path to the destination. The algorithm finds the set of such
paths that have the smallest cost.
Looking at this another way, inter-area routing can be pictured as
forcing a star configuration on the Autonomous System, with the backbone
as hub and and each of the areas as spokes.
The topology of the backbone dictates the backbone paths used between
areas. The topology of the backbone can be enhanced by adding virtual
links. This gives the system administrator some control over the routes
taken by inter-area traffic.
The correct area border router to use as the packet exits the source
area is chosen in exactly the same way routers advertising external
routes are chosen. Each area border router in an area summarizes for
the area its cost to all networks external to the area. After the SPF
tree is calculated for the area, routes to all other networks are
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RFC 1247 OSPF Version 2 July 1991
calculated by examining the summaries of the area border routers.
3.3 Classification of routers
Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as router RT5 in Figure 2. When the AS is split into
OSPF areas, the routers are further divided according to function into
the following four overlapping categories:
Internal routers
A router with all directly connected networks belonging to the same
area. Routers with only backbone interfaces also belong to this
category. These routers run a single copy of the basic routing
algorithm.
Area border routers
A router that attaches to multiple areas. Area border routers run
multiple copies of the basic algorithm, one copy for each attached
area and an additional copy for the backbone. Area border routers
condense the topological information of their attached areas for
distribution to the backbone. The backbone in turn distributes the
information to the other areas.
Backbone routers
A router that has an interface to the backbone. This includes all
routers that interface to more than one area (i.e., area border
routers). However, backbone routers do not have to be area border
routers. Routers with all interfaces connected to the backbone are
considered to be internal routers.
AS boundary routers
A router that exchanges routing information with routers belonging
to other Autonomous Systems. Such a router has AS external routes
that are advertised throughout the Autonomous System. The path to
each AS boundary router is known by every router in the AS. This
classification is completely independent of the previous
classifications: AS boundary routers may be internal or area border
routers, and may or may not participate in the backbone.
3.4 A sample area configuration
Figure 6 shows a sample area configuration. The first area consists of
networks N1-N4, along with their attached routers RT1-RT4. The second
area consists of networks N6-N8, along with their attached routers RT7,
[Moy] [Page 16]RFC 1247 OSPF Version 2 July 1991
RT8, RT10, RT11. The third area consists of networks N9-N11 and host
H1, along with their attached routers RT9, RT11, RT12. The third area
has been configured so that networks N9-N11 and host H1 will all be
grouped into a single route, when advertised external to the area (see
Section 3.5 for more details).
In Figure 6, routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are internal
routers. Routers RT3, RT4, RT7, RT10 and RT11 are area border routers.
Finally as before, routers RT5 and RT7 are AS boundary routers.
Figure 7 shows the resulting topological database for the Area 1. The
figure completely describes that area's intra-area routing. It also
shows the complete view of the internet for the two internal routers RT1
and RT2. It is the job of the area border routers, RT3 and RT4, to
advertise into Area 1 the distances to all destinations external to the
area. These are indicated in Figure 7 by the dashed stub routes. Also,
RT3 and RT4 must advertise into Area 1 the location of the AS boundary
routers RT5 and RT7. Finally, external advertisements from RT5 and RT7
are flooded throughout the entire AS, and in particular throughout Area
1. These advertisements are included in Area 1's database, and yield
routes to networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone. Their backbone advertisements are shown
in Table 4. These summaries show which networks are contained in Area 1
(i.e., networks N1-N4), and the distance to these networks from the
routers RT3 and RT4 respectively.
The topological database for the backbone is shown in Figure 8. The set
of routers pictured are the backbone routers. Router RT11 is a backbone
router because it belongs to two areas. In order to make the backbone
connected, a virtual link has been configured between routers R10 and
R11.
__________________________________________
(Figure not included in text version.)
Figure 6: A sample OSPF area configuration
__________________________________________
[Moy] [Page 17]RFC 1247 OSPF Version 2 July 1991
Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone by routers RT3 and RT4.
______________________________________
(Figure not included in text version.)
Figure 7: Area 1's Database
Figure 8: The backbone database
______________________________________
Again, routers RT3, RT4, RT7, RT10 and RT11 are area border routers. As
routers RT3 and RT4 did above, they have condensed the routing
information of their attached areas for distribution via the backbone;
these are the dashed stubs that appear in Figure 8. Remember that the
third area has been configured to condense networks N9-N11 and Host H1
into a single route. This yields a single dashed line for networks N9-
N11 and Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary
routers; their externally derived information also appears on the graph
in Figure 8 as stubs.
The backbone enables the exchange of summary information between area
border routers. Every area border router hears the area summaries from
all other area border routers. It then forms a picture of the distance
to all networks outside of its area by examining the collected
advertisements, and adding in the backbone distance to each advertising
router.
Again using routers RT3 and RT4 as an example, the procedure goes as
follows: They first calculate the SPF tree for the backbone. This gives
the distances to all other area border routers. Also noted are the
distances to networks (Ia and Ib) and AS boundary routers (RT5 and RT7)
that belong to the backbone. This calculation is shown in Table 5.
Next, by looking at the area summaries from these area border routers,
RT3 and RT4 can determine the distance to all networks outside their
[Moy] [Page 18]RFC 1247 OSPF Version 2 July 1991
Area border dist from dist from
router RT3 RT4
______________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
______________________________________
to Ia 20 27
to Ib 15 22
______________________________________
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated by routers RT3 and RT4.
area. These distances are then advertised internally to the area by RT3
and RT4. The advertisements that router RT3 and RT4 will make into Area
1 are shown in Table 6. Note that Table 6 assumes that an area range
has been configured for the backbone which groups I5 and I6 into a
single advertisement.
The information imported into Area 1 by routers RT3 and RT4 enables an
internal router, such as RT1, to choose an area border router
intelligently. Router RT1 would use RT4 for traffic to network N6, RT3
for traffic to network N10, and would load share between the two for
Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 15 22
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 19 26
_________________________________
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1 by routers RT3 and RT4.
[Moy] [Page 19]RFC 1247 OSPF Version 2 July 1991
traffic to network N8.
Router RT1 can also determine in this manner the shortest path to the AS
boundary routers RT5 and RT7. Then, by looking at RT5 and RT7's
external advertisements, router RT1 can decide between RT5 or RT7 when
sending to a destination in another Autonomous System (one of the
networks N12-N15).
Note that a failure of the line between routers RT6 and RT10 will cause
the backbone to become disconnected. Configuring another virtual link
between routers RT7 and RT10 will give the backbone more connectivity
and more resistance to such failures. Also, a virtual link between RT7
and RT10 would allow a much shorter path between the third area
(containing N9) and the router RT7, which is advertising a good route to
external network N12.
3.5 IP subnetting support
OSPF attaches an IP address mask to each advertised route. The mask
indicates the range of addresses being described by the particular
route. For example, a summary advertisement for the destination
128.185.0.0 with a mask of 0xffff0000 actually is describing a single
route to the collection of destinations 128.185.0.0 - 128.185.255.255.
Similarly, host routes are always advertised with a mask of 0xffffffff,
indicating the presence of only a single destination.
Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-length subnet
masks. This means that a single IP class A, B, or C network number can
be broken up into many subnets of various sizes. For example, the
network 128.185.0.0 could be broken up into 64 variable-sized subnets:
16 subnets of size 4K, 16 subnets of size 256, and 32 subnets of size 8.
Table 7 shows some of the resulting network addresses together with
their masks:
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8
Table 7: Some sample subnet sizes.
[Moy] [Page 20]RFC 1247 OSPF Version 2 July 1991
There are many possible ways of dividing up a class A, B, and C network
into variable sized subnets. The precise procedure for doing so is
beyond the scope of this specification. The specification however
establishes the following guideline: When an IP packet is forwarded, it
is always forwarded to the network that is the best match for the
packet's destination. Here best match is synonymous with the longest or
most specific match. For example, the default route with destination of
0.0.0.0 and mask 0x00000000 is always a match for every IP destination.
Yet it is always less specific than any other match. Subnet masks must
be assigned so that the best match for any IP destination is
unambiguous.
The OSPF area concept is modelled after an IP subnetted network. OSPF
areas have been loosely defined to be a collection of networks. In
actuality, an OSPF area is specified to be a list of address ranges (see
Section C.2 for more details). Each address range is defined as an
[address,mask] pair. Many separate networks may then be contained in a
single address range, just as a subnetted network is composed of many
separate subnets. Area border routers then summarize the area contents
(for distribution to the backbone) by advertising a single route for
each address range. The cost of the route is the minimum cost to any of
the networks falling in the specified range.
For example, an IP subnetted network can be configured as a single OSPF
area. In that case, the area would be defined as a single address
range: a class A, B, or C network number along with its natural IP mask.
Inside the area, any number of variable sized subnets could be defined.
External to the area, a single route for the entire subnetted network
would be distributed, hiding even the fact that the network is subnetted
at all. The cost of this route is the minimum of the set of costs to
the component subnets.
3.6 Supporting stub areas
In some Autonomous Systems, the majority of the topological database may
consist of external advertisements. An OSPF external advertisement is
usually flooded throughout the entire AS. However, OSPF allows certain
areas to be configured as "stub areas". External advertisements are not
flooded into/throughout stub areas; routing to AS external destinations
in these areas is based on a (per-area) default only. This reduces the
topological database size, and therefore the memory requirements, for a
stub area's internal routers.
In order to take advantage of the OSPF stub area support, default
routing must be used in the stub area. This is accomplished as follows.
One or more of the stub area's area border routers must advertise a
default route into the stub area via summary advertisements. These
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summary defaults are flooded throughout the stub area, but no further.
(For this reason these defaults pertain only to the particular stub
area). These summary default routes will match any destination that is
not explicitly reachable by an intra-area or inter-area path (i.e., AS
external destinations).
An area can be configured as stub when there is a single exit point from
the area, or when the choice of exit point need not be made on a per-
external-destination basis. For example, area 3 in Figure 6 could be
configured as a stub area, because all external traffic must travel
though its single area border router RT11. If area 3 were configured as
a stub, router RT11 would advertise a default route for distribution
inside area 3 (in a summary advertisement), instead of flooding the
external advertisements for networks N12-N15 into/throughout the area.
The OSPF protocol ensures that all routers belonging to an area agree on
whether the area has been configured as a stub. This guarantees that no
confusion will arise in the flooding of external advertisements.
There are a couple of restrictions on the use of stub areas. Virtual
links cannot be configured through stub areas. In addition, AS boundary
routers cannot be placed internal to stub areas.
3.7 Partitions of areas
OSPF does not actively attempt to repair area partitions. When an area
becomes partitioned, each component simply becomes a separate area. The
backbone then performs routing between the new areas. Some destinations
reachable via intra-area routing before the partition will now require
inter-area routing.
In the previous section, an area was described as a list of address
ranges. Any particular address range must still be completely contained
in a single component of the area partition. This has to do with the
way the area contents are summarized to the backbone. Also, the
backbone itself must not partition. If it does, parts of the Autonomous
System will become unreachable. Backbone partitions can be repaired by
configuring virtual links (see Section 15).
Another way to think about area partitions is to look at the Autonomous
System graph that was introduced in Section 2. Area IDs can be viewed
as colors for the graph's edges.[1] Each edge of the graph connects to a
network, or is itself a point-to-point network. In either case, the
edge is colored with the network's Area ID.
A group of edges, all having the same color, and interconnected by
vertices, represents an area. If the topology of the Autonomous System
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is intact, the graph will have several regions of color, each color
being a distinct Area ID.
When the AS topology changes, one of the areas may become partitioned.
The graph of the AS will then have multiple regions of the same color
(Area ID). The routing in the Autonomous System will continue to
function as long as these regions of same color are connected by the
single backbone region.
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RFC 1247 OSPF Version 2 July 1991
4. Functional Summary
A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of the
algorithm. A brief summary of the routing algorithm follows.
When a router starts, it first initializes the routing protocol data
structures. The router then waits for indications from the lower-level
protocols that its interfaces are functional.
A router then uses the OSPF's Hello Protocol to acquire neighbors. The
router sends Hello packets to its neighbors, and in turn receives their
Hello packets. On broadcast and point-to-point networks, the router
dynamically detects its neighboring routers by sending its Hello packets
to the multicast address AllSPFRouters. On non-broadcast networks, some
configuration information is necessary in order to discover neighbors.
On all multi-access networks (broadcast or non-broadcast), the Hello
Protocol also elects a Designated router for the network.
The router will attempt to form adjacencies with some of its newly
acquired neighbors. Topological databases are synchronized between
pairs of adjacent routers. On multi-access networks, the Designated
Router determines which routers should become adjacent.
Adjacencies control the distribution of routing protocol packets.
Routing protocol packets are sent and received only on adjacencies. In
particular, distribution of topological database updates proceeds along
adjacencies.
A router periodically advertises its state, which is also called link
state. Link state is also advertised when a router's state changes. A
router's adjacencies are reflected in the contents of its link state
advertisements. This relationship between adjacencies and link state
allows the protocol to detect dead routers in a timely fashion.
Link state advertisements are flooded throughout the area. The flooding
algorithm is reliable, ensuring that all routers in an area have exactly
the same topological database. This database consists of the collection
of link state advertisements received from each router belonging to the
area. From this database each router calculates a shortest-path tree,
with itself as root. This shortest-path tree in turn yields a routing
table for the protocol.
4.1 Inter-area routing
The previous section described the operation of the protocol within a
single area. For intra-area routing, no other routing information is
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RFC 1247 OSPF Version 2 July 1991
pertinent. In order to be able to route to destinations outside of the
area, the area border routers inject additional routing information into
the area. This additional information is a distillation of the rest of
the Autonomous System's topology.
This distillation is accomplished as follows: Each area border router is
by definition connected to the backbone. Each area border router
summarizes the topology of its attached areas for transmission on the
backbone, and hence to all other area border routers. A area border
router then has complete topological information concerning the
backbone, and the area summaries from each of the other area border
routers. From this information, the router calculates paths to all
destinations not contained in its attached areas. The router then
advertises these paths to its attached areas. This enables the area's
internal routers to pick the best exit router when forwarding traffic to
destinations in other areas.
4.2 AS external routes
Routers that have information regarding other Autonomous Systems can
flood this information throughout the AS. This external routing
information is distributed verbatim to every participating router.
There is one exception: external routing information is not flooded into
"stub" areas (see Section 3.6).
To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas). For that reason, the locations of these AS
boundary routers are summarized by the (non-stub) area border routers.
4.3 Routing protocol packets
The OSPF protocol runs directly over IP, using IP protocol 89. OSPF
does not provide any explicit fragmentation/reassembly support. When
fragmentation is necessary, IP fragmentation/reassembly is used. OSPF
protocol packets have been designed so that large protocol packets can
generally be split into several smaller protocol packets. This practice
is recommended; IP fragmentation should be avoided whenever possible.
Routing protocol packets should always be sent with the IP TOS field set
to 0. If at all possible, routing protocol packets should be given
preference over regular IP data traffic, both when being sent and
received. As an aid to accomplishing this, OSPF protocol packets should
have their IP precedence field set to the value Internetwork Control
(see [RFC 791]).
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RFC 1247 OSPF Version 2 July 1991
All OSPF protocol packets share a common protocol header that is
described in Appendix A. The OSPF packet types are listed below in
Table 8. Their formats are also described in Appendix A.
Type Packet name Protocol function
__________________________________________________________
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment
Table 8: OSPF packet types.
OSPF's Hello protocol uses Hello packets to discover and maintain
neighbor relationships. The Database Description and Link State Request
packets are used in the forming of adjacencies. OSPF's reliable update
mechanism is implemented by the Link State Update and Link State
Acknowledgment packets.
Each Link State Update packet carries a set of new link state
advertisements one hop further away from their point of origination. A
single Link State Update packet may contain the link state
advertisements of several routers. Each advertisement is tagged with
the ID of the originating router and a checksum of its link state
contents. The five different types of OSPF link state advertisements
are listed below in Table 9.
LS Advertisement Advertisement description
type name
____________________________________________________________________________
1 Router links advs. Originated by all routers. This
advs. advertisement describes the collected
states of the router's interfaces to an
area. Flooded throughout a single area
only.
____________________________________________________________________________
2 Network links Originated for multi-access networks by
advs. the Designated Router. This
advertisement contains the list of
routers connected to the network.
Flooded throughout a single area only.
[Moy] [Page 26]RFC 1247 OSPF Version 2 July 1991
LS Advertisement Advertisement description
type name
____________________________________________________________________________
____________________________________________________________________________
3,4 Summary link Originated by area border routers, and
advs. flooded throughout their associated
area. Each summary link advertisement
describes a route to a destination
outside the area, yet still inside the
AS (i.e., an inter-area route). Type 3
advertisements describe routes to
networks. Type 4 advertisements
describe routes to AS boundary routers.
____________________________________________________________________________
5 AS external Originated by AS boundary routers, and
link advs. flooded throughout the AS. Each external
advertisement describes a route to a
destination in another Autonomous
System. Default routes for the AS can
also be described by AS external advertisements.
Table 9: OSPF link state advertisements.
As mentioned above, OSPF routing packets (with the exception of Hellos)
are sent only over adjacencies. Note that this means that all protocol
packets travel a single IP hop, except those that are sent over virtual
adjacencies. The IP source address of an OSPF protocol packet is one
end of a router adjacency, and the IP destination address is either the
other end of the adjacency or an IP multicast address.
4.4 Basic implementation requirements
An implementation of OSPF requires the following pieces of system
support:
Timers
Two different kind of timers are required. The first kind, called
single shot timers, fire once and cause a protocol event to be
processed. The second kind, called interval timers, fire at
continuous intervals. These are used for the sending of packets at
regular intervals. A good example of this is the regular broadcast
of Hello packets (on broadcast networks). The granularity of both
kinds of timers is one second.
Interval timers should be implemented to avoid drift. In some
[Moy] [Page 27]RFC 1247 OSPF Version 2 July 1991
router implementations, packet processing can affect timer
execution. When multiple routers are attached to a single network,
all doing broadcasts, this can lead to the synchronization of
routing packets (which should be avoided). If timers cannot be
implemented to avoid drift, small random amounts should be added
to/subtracted from the timer interval at each firing.
IP multicast
Certain OSPF packets use IP multicast. Support for receiving and
sending IP multicasts, along with the appropriate lower-level
protocol support, is required. These IP multicast packets never
travel more than one hop. For information on IP multicast, see [RFC
1112].
Lower-level protocol support
The lower level protocols referred to here are the network access
protocols, such as the Ethernet data link layer. Indications must
be passed from from these protocols to OSPF as the network interface
goes up and down. For example, on an ethernet it would be valuable
to know when the ethernet transceiver cable becomes unplugged.
Non-broadcast lower-level protocol support
Remember that non-broadcast networks are multi-access networks such
as a X.25 PDN. On these networks, the Hello Protocol can be aided
by providing an indication to OSPF when an attempt is made to send a
packet to a dead or non-existent router. For example, on a PDN a
dead router may be indicated by the reception of a X.25 clear with
an appropriate cause and diagnostic, and this information would be
passed to OSPF.
List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of link state advertisements. For example, the
advertisements that will be retransmitted to an adjacent router
until acknowledged are described as a list. Any particular
advertisement may be on many such lists. An OSPF implementation
needs to be able to manipulate these lists, adding and deleting
constituent advertisements as necessary.
Tasking support
Certain procedures described in this specification invoke other
procedures. At times, these other procedures should be executed
in-line, that is, before the current procedure is finished. This is
indicated in the text by instructions to execute a procedure. At
other times, the other procedures are to be executed only when the
current procedure has finished. This is indicated by instructions
to schedule a task.
[Moy] [Page 28]RFC 1247 OSPF Version 2 July 1991
4.5 Optional OSPF capabilities
The OSPF protocol defines several optional capabilities. A router
indicates the optional capabilities that it supports in its OSPF Hello
packets, Database Description packets and in its link state
advertisements. This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System.
Some capabilities must be supported by all routers attached to a
specific area. In this case, a router will not accept a neighbor's
Hello unless there is a match in reported capabilities (i.e., a
capability mismatch prevents a neighbor relationship from forming). An
example of this is the external routing capability (see below).
Other capabilities can be negotiated during the database synchronization
process. This is accomplished by specifying the optional capabilities
in Database Description packets. A capability mismatch with a neighbor
is this case will result in only a subset of link state advertisements
being exchanged between the two neighbors.
The routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since the
optional capabilities are reported in link state advertisements, routers
incapable of certain functions can be avoided when building the shortest
path tree. An example of this is the TOS routing capability (see
below).
The current OSPF optional capabilities are listed below. See Section
A.2 for more information.
External routing capability
Entire OSPF areas can be configured as "stubs" (see Section 3.6).
AS external advertisements will not be flooded into stub areas.
This capability is represented by the E-bit in the OSPF options
field (see Section A.2). In order to ensure consistent
configuration of stub areas, all routers interfacing to such an area
must have the E-bit clear in their Hello packets (see Sections 9.5
and 10.5).
TOS capability
All OSPF implementations must be able to calculate separate routes
based on IP Type of Service. However, to save routing table space
and processing resources, an OSPF router can be configured to ignore
TOS when forwarding packets. In this case, the router calculates
routes for TOS 0 only. This capability is represented by the T-bit
in the OSPF options field (see Section A.2). TOS-capable routers
will attempt to avoid non-TOS-capable routers when calculating non-
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zero TOS paths.
5. Protocol Data Structures
The OSPF protocol is described in this specification in terms of its
operation on various protocol data structures. The following list
comprises the top-level OSPF data structures. Any initialization that
needs to be done is noted. Areas, OSPF interfaces and neighbors also
have associated data structures that are described later in this
specification.
Router ID
a 32-bit number that uniquely identifies this router in the AS. One
possible implementation strategy would be to use the smallest IP
interface address belonging to the router.
Pointers to area structures
Each one of the areas to which the router is connected has its own
data structure. This data structure describes the working of the
basic algorithm. Remember that each area runs a separate copy of
the basic algorithm.
Pointer to the backbone structure
The basic algorithm operates on the backbone as if it were an area.
For this reason the backbone is represented as an area structure.
Virtual links configured
The virtual links configured with this router as one endpoint. In
order to have configured virtual links, the router itself must be an
area border router. Virtual links are identified by the Router ID
of the other endpoint -- which is another area border router. These
two endpoint routers must be attached to a common area, called the
virtual link's transit area. Virtual links are part of the
backbone, and behave as if they were unnumbered point-to-point
networks between the two routers. A virtual link uses the intra-
area routing of its transit area to forward packets. Virtual links
are brought up and down through the building of the shortest-path
trees for the transit area.
List of external routes
These are routes to destinations external to the Autonomous System,
that have been gained either through direct experience with another
routing protocol (such as EGP), or through configuration
information, or through a combination of the two (e.g., dynamic
external info. to be advertised by OSPF with configured metric).
Any router having these external routes is called an AS boundary
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router. These routes are advertised by the router to the entire AS
through AS external link advertisements.
List of AS external link advertisements
Part of the topological database. These have have originated from
the AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS external link
advertisements have been self originated.
The routing table
Derived from the topological database. Each destination that the
router can forward to is represented by a cost and a set of paths.
A path is described by its type and next hop. For more information,
see Section 11.
TOS capability
This item indicates whether the router will calculate separate
routes based on TOS. This is a configurable parameter. For more
information, see Sections 4.5 and 16.9.
Figure 9 shows the collection of data structures present in a typical
router. The router pictured is RT10, from the map in Figure 6. Note
that router RT10 has a virtual link configured to router RT11, with Area
2 as the link's transit area. This is indicated by the dashed line in
Figure 9. When the virtual link becomes active, through the building of
the shortest path tree for Area 2, it becomes an interface to the
backbone (see the two backbone interfaces depicted in Figure 9).
6. The Area Data Structure
The area data structure contains all the information used to run the
basic routing algorithm. Remember that each area maintains its own
topological database. Router interfaces and adjacencies belong to a
_______________________________________
(Figure not included in text version.)
Figure 9: Router RT10's Data Structures
_______________________________________
[Moy] [Page 31]RFC 1247 OSPF Version 2 July 1991
single area.
The backbone has all the properties of an area. For that reason it is
also represented by an area data structure. Note that some items in the
structure apply differently to the backbone than to areas.
The area topological (or link state) database consists of the collection
of router links, network links and summary links advertisements that
have originated from the area's routers. This information is flooded
throughout a single area only. The list of AS external advertisements
is also considered to be part of each area's topological database.
Area ID
A 32-bit number identifying the area. 0 is reserved for the area ID
of the backbone. If assigning subnetted networks as separate areas,
the IP network number could be used as the Area ID.
List of component address ranges
The address ranges that define the area. Each address range is
specified by an [address,mask] pair. Each network is then assigned
to an area depending on the address range that it falls into
(specified address ranges are not allowed to overlap). As an
example, if an IP subnetted network is to be its own separate OSPF
area, the area is defined to consist of a single address range - an
IP network number with its natural (class A, B or C) mask.
Associated router interfaces
This router's interfaces connecting to the area. A router interface
belongs to one and only one area (or the backbone). For the
backbone structure this list includes all the virtual adjacencies.
A virtual adjacency is identified by the router ID of its other
endpoint; its cost is the cost of the shortest intra-area path that
exists between the two routers.
List of router links advertisements
A router links advertisement is generated by each router in the
area. It describes the state of the router's interfaces to the
area.
List of network links advertisements
One network links advertisement is generated for each transit
multi-access network in the area. It describes the set of routers
currently connected to the network.
List of summary links advertisements
Summary link advertisements originate from the area's area border
routers. They describe routes to destinations internal to the
[Moy] [Page 32]RFC 1247 OSPF Version 2 July 1991
Autonomous System, yet external to the area.
Shortest-path tree
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router links and network links
advertisements by the Dijkstra algorithm.
Authentication type
The type of authentication used for this area. Authentication types
are defined in Appendix E. All OSPF packet exchanges are
authenticated. Different authentication schemes may be used in
different areas.
External routing capability
Whether AS external advertisements will be flooded into/throughout
the area. This is a configurable parameter. If AS external
advertisements are excluded from the area, the area is called a
"stub". Internal to stub areas, routing to external destinations
will be based solely on a default summary route. The backbone
cannot be configured as a stub area. Also, virtual links cannot be
configured through stub areas. For more information, see Section
3.6.
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost indicates
the cost of the default summary link that the router should
advertise into the area. There can be a separate cost configured
for each IP TOS. See Section 12.4.3 for more information.
Unless otherwise specified, the remaining sections of this document
refer to the operation of the protocol in a single area.
7. Bringing Up Adjacencies
OSPF creates adjacencies between neighboring routers for the purpose of
exchanging routing information. Not every two neighboring routers will
become adjacent. This section covers the generalities involved in
creating adjacencies. For further details consult Section 10.
7.1 The Hello Protocol
The Hello Protocol is responsible for establishing and maintaining
neighbor relationships. It also ensures that communication between
neighbors is bidirectional. Hello packets are sent periodically out all
[Moy] [Page 33]RFC 1247 OSPF Version 2 July 1991
router interfaces. Bidirectional communication is indicated when the
router sees itself listed in the neighbor's Hello Packet.
On multi-access networks, the Hello Protocol elects a Designated Router
for the network. Among other things, the Designated Router controls
what adjacencies will be formed over the network (see below).
The Hello Protocol works differently on broadcast networks, as compared
to non-broadcast networks. On broadcast networks, each router
advertises itself by periodically multicasting Hello Packets. This
allows neighbors to be discovered dynamically. These Hello Packets
contain the router's view of the Designated Router's identity, and the
list of routers whose Hellos have been seen recently.
On non-broadcast networks some configuration information is necessary
for the operation of the Hello Protocol. Each router that may
potentially become Designated Router has a list of all other routers
attached to the network. A router, having Designated Router potential,
sends hellos to all other potential Designated Routers when its
interface to the non-broadcast network first becomes operational. This
is an attempt to find the Designated Router for the network. If the
router itself is elected Designated Router, it begins sending hellos to
all other routers attached to the network.
After a neighbor has been discovered, bidirectional communication
ensured, and (if on a multi-access network) a Designated Router elected,
a decision is made regarding whether or not an adjacency should be
formed with the neighbor (see Section 10.4). An attempt is always made
to establish adjacencies over point-to-point networks and virtual links.
The first step in bringing up an adjacency is to synchronize the
neighbors' topological databases. This is covered in the next section.
7.2 The Synchronization of Databases
In an SPF-based routing algorithm, it is very important for all routers'
topological databases to stay synchronized. OSPF simplifies this by
requiring only adjacent routers to remain synchronized. The
synchronization process begins as soon as the routers attempt to bring
up the adjacency. Each router describes its database by sending a
sequence of Database Description packets to its neighbor. Each Database
Description Packet describes a set of link state advertisements
belonging to the database. When the neighbor sees a link state
advertisement that is more recent than its own database copy, it makes a
note that this newer advertisement should be requested.
This sending and receiving of Database Description packets is called the
"Database Exchange Process". During this process, the two routers form
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a master/slave relationship. Each Database Description Packet has a
sequence number. Database Description Packets sent by the master
(polls) are acknowledged by the slave through echoing of the sequence
number. Both polls and their responses contain summaries of link state
data. The master is the only one allowed to retransmit Database
Description Packets. It does so only at fixed intervals, the length of
which is the configured constant RxmtInterval.
Each Database Description contains an indication that there are more
packets to follow --- the M-bit. The Database Exchange Process is over
when a router has received and sent Database Description Packets with
the M-bit off.
During and after the Database Exchange Process, each router has a list
of those link state advertisements for which the neighbor has more up-
to-date instances. These advertisements are requested in Link State
Request Packets. Link State Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval. When the
Database Description Process has completed and all Link State Requests
have been satisfied, the databases are deemed synchronized and the
routers are marked fully adjacent. At this time the adjacency is fully
functional and is advertised in the two routers' link state
advertisements.
The adjacency is used by the flooding procedure as soon as the Database
Exchange Process begins. This simplifies database synchronization, and
guarantees that it finishes in a predictable period of time.
7.3 The Designated Router
Every multi-access network has a Designated Router. The Designated
Router performs two main functions for the routing protocol:
o The Designated Router originates a network links advertisement on
behalf of the network. This advertisement lists the set of routers
(including the Designated Router itself) currently attached to the
network. The Link State ID for this advertisement (see Section
12.1.4) is the IP interface address of the Designated Router. The
IP network number can then be obtained by using the subnet/network
mask.
o The Designated router becomes adjacent to all other routers on the
network. Since the link state databases are synchronized across
adjacencies (through adjacency bring-up and then the flooding
procedure), the Designated Router plays a central part in the
synchronization process.
[Moy] [Page 35]RFC 1247 OSPF Version 2 July 1991
The Designated Router is elected by the Hello Protocol. A router's
Hello Packet contains its Router Priority, which is configurable on a
per-interface basis. In general, when a router's interface to a network
first becomes functional, it checks to see whether there is currently a
Designated Router for the network. If there is, it accepts that
Designated Router, regardless of its Router Priority. (This makes it
harder to predict the identity of the Designated Router, but ensures
that the Designated Router changes less often. See below.) Otherwise,
the router itself becomes Designated Router if it has the highest Router
Priority on the network. A more detailed (and more accurate)
description of Designated Router election is presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In order to
optimize the flooding procedure on broadcast networks, the Designated
Router multicasts its Link State Update Packets to the address
AllSPFRouters, rather than sending separate packets over each adjacency.
Section 2 of this document discusses the directed graph representation
of an area. Router nodes are labelled with their Router ID. Broadcast
network nodes are actually labelled with the IP address of their
Designated Router. It follows that when the Designated Router changes,
it appears as if the network node on the graph is replaced by an
entirely new node. This will cause the network and all its attached
routers to originate new link state advertisements. Until the
topological databases again converge, some temporary loss of
connectivity may result. This may result in ICMP unreachable messages
being sent in response to data traffic. For that reason, the Designated
Router should change only infrequently. Router Priorities should be
configured so that the most dependable router on a network eventually
becomes Designated Router.
7.4 The Backup Designated Router
In order to make the transition to a new Designated Router smoother,
there is a Backup Designated Router for each multi-access network. The
Backup Designated Router is also adjacent to all routers on the network,
and becomes Designated Router when the previous Designated Router fails.
If there were no Backup Designated Router, when a new Designated Router
became necessary, new adjacencies would have to be formed between the
router and all other routers attached to the network. Part of the
adjacency forming process is the synchronizing of topological databases,
which can potentially take quite a long time. During this time, the
network would not be available for transit data traffic. The Backup
Designated obviates the need to form these adjacencies, since they
already exist. This means the period of disruption in transit traffic
lasts only as long as it take to flood the new link state advertisements
(which announce the new Designated Router).
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RFC 1247 OSPF Version 2 July 1991
The Backup Designated Router does not generate a network links
advertisement for the network. (If it did, the transition to a new
Designated Router would be even faster. However, this is a tradeoff
between database size and speed of convergence when the Designated
Router disappears.)
The Backup Designated Router is also elected by the Hello Protocol.
Each Hello Packet has a field that specifies the Backup Designated
Router for the network.
In some steps of the flooding procedure, the Backup Designated Router
plays a passive role, letting the Designated Router do more of the work.
This cuts down on the amount of local routing traffic. See Section 13.3
for more information.
7.5 The graph of adjacencies
An adjacency is bound to the network that the two routers have in
common. If two routers have multiple networks in common, they may have
multiple adjacencies between them.
One can picture the collection of adjacencies on a network as forming an
undirected graph. The vertices consist of routers, with an edge joining
two routers if they are adjacent. The graph of adjacencies describes
the flow of routing protocol packets, and in particular Link State
Updates, through the Autonomous System.
Two graphs are possible, depending on whether the common network is
multi-access. On physical point-to-point networks (and virtual links),
the two routers joined by the network will be adjacent after their
databases have been synchronized. On multi-access networks, both the
Designated Router and the Backup Designated Router are adjacent to all
other routers attached to the network, and these account for all
adjacencies.
These graphs are shown in Figure 10. It is assumed that router RT7 has
become the Designated Router, and router RT3 the Backup Designated
Router, for the network N2. The Backup Designated Router performs a
lesser function during the flooding procedure than the Designated Router
(see Section 13.3). This is the reason for the dashed lines connecting
the Backup Designated Router RT3.
8. Protocol Packet Processing
This section discusses the general processing of routing protocol
packets. It is very important that the router topological databases
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RFC 1247 OSPF Version 2 July 1991
remain synchronized. For this reason, routing protocol packets should
get preferential treatment over ordinary data packets, both in sending
and receiving.
Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the adjacencies).
This means that all protocol packets travel a single IP hop, except
those sent over virtual links.
All routing protocol packets begin with a standard header. The sections
below give the details on how to fill in and verify this standard
header. Then, for each packet type, the section is listed that gives
more details on that particular packet type's processing.
8.1 Sending protocol packets
When a router sends a routing protocol packet, it fills in the fields of
that standard header as follows. For more details on the header format
consult Section A.3.1:
Version #
Set to 2, the version number of the protocol as documented in this
specification.
Packet type
The type of OSPF packet, such as Link state Update or Hello Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard header.
Router ID
The identity of the router itself (who is originating the packet).
______________________________________
(Figure not included in text version.)
Figure 10: The graph of adjacencies
Figure 11: Interface state changes
______________________________________
[Moy] [Page 38]RFC 1247 OSPF Version 2 July 1991
Area ID
The area that the packet is being sent into.
Checksum
The standard IP 16-bit one's complement checksum of the entire OSPF
packet, excluding the 64-bit authentication field. This checksum
should be calculated before handing the packet to the appropriate
authentication procedure.
Autype and Authentication
Each OSPF packet exchange is authenticated. Authentication types
are assigned by the protocol and documented in Appendix E. A
different authentication scheme can be used for each OSPF area. The
64-bit authentication field is set by the appropriate authentication
procedure (determined by Autype). This procedure should be the last
called when forming the packet to be sent. The setting of the
authentication field is determined by the packet contents and the
authentication key (which is configurable on a per-interface basis).
The IP destination address for the packet is selected as follows. On
physical point-to-point networks, the IP destination is always set to
the the address AllSPFRouters. On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts, i.e.,
sent directly to the other end of the adjacency. In this case, the IP
destination is just the neighbor IP address associated with the other
end of the adjacency (see Section 10). The only packets not sent as
unicasts are on broadcast networks; on these networks Hello packets are
sent to the multicast destination AllSPFRouters, the Designated Router
and its Backup send both Link State Update Packets and Link State
Acknowledgment Packets to the multicast address AllSPFRouters, while all
other routers send both their Link State Update and Link State
Acknowledgment Packets to the multicast address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as
unicasts.
The IP source address should be set to the IP address of the sending
interface. Interfaces to unnumbered point-to-point networks have no
associated IP address. On these interfaces, the IP source should be set
to any of the other IP addresses belonging to the router. For this
reason, there must be at least one IP address assigned to the router.[2]
Note that, for most purposes, virtual links act precisely the same as
unnumbered point-to-point networks. However, each virtual link does
have an interface IP address (discovered during the routing table build
process) which is used as the IP source when sending packets over the
virtual link.
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For more information on the format of specific packet types, consult the
sections listed in Table 10.
Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing packet transmission.
8.2 Receiving protocol packets
Whenever a protocol packet is received by the router it is marked with
the interface it was received on. For routers that have virtual links
configured, it may not be immediately obvious which interface to
associate the packet with. For example, consider the router RT11
depicted in Figure 6. If RT11 receives an OSPF protocol packet on its
interface to network N8, it may want to associate the packet with the
interface to area 2, or with the virtual link to router RT10 (which is
part of the backbone). In the following, we assume that the packet is
initially associated with the non-virtual link.[3]
In order for the packet to be accepted at the IP level, it must pass a
number of tests, even before the packet is passed to OSPF for
processing:
o The IP checksum must be correct.
o The packet's IP destination address must be the IP address of the
receiving interface, or one of the IP multicast addresses
AllSPFRouters or AllDRouters.
o The IP protocol specified must be OSPF (89).
o Locally originated packets should not be passed on to OSPF. That
is, the source IP address should be examined to make sure this is
not a multicast packet that the router itself generated.
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Next, the OSPF packet header is verified. The fields specified in the
header must match those configured for the receiving interface. If they
do not, the packet should be discarded:
o The version number field must specify protocol version 2.
o The 16-bit checksum of the OSPF packet's contents must be verified.
Remember that the 64-bit authentication field must be excluded from
the checksum calculation.
o The Area ID found in the OSPF header must be verified. If both of
the following cases fail, the packet should be discarded. The Area
ID specified in the header must either:
(1) Match the Area ID of the receiving interface. In this case, the
packet has been sent over a single hop. Therefore, the packet's
IP source address must be on the same network as the receiving
interface. This can be determined by comparing the packet's IP
source address to the interface's IP address, after masking both
addresses with the interface mask.
(2) Indicate the backbone. In this case, the packet has been sent
over a virtual link. The receiving router must be an area
border router, and the router ID specified in the packet (the
source router) must be the other end of a configured virtual
link. The receiving interface must also attach to the virtual
link's configured transit area. If all of these checks succeed,
the packet is accepted and is from now on associated with the
virtual link (and the backbone area).
o Packets whose IP destination is AllDRouters should only be accepted
if the state of the receiving interface is DR or Backup (see Section
9.1).
o The Authentication type specified must match the authentication type
specified for the associated area.
Next, the packet must be authenticated. This depends on the
authentication type specified (see Appendix E). The authentication
procedure may use an Authentication key, which can be configured on a
per-interface basis. If the authentication fails, the packet should be
discarded.
If the packet type is Hello, it should then be further processed by the
Hello Protocol (see Section 10.5). All other packet types are
sent/received only on adjacencies. This means that the packet must have
[Moy] [Page 41]RFC 1247 OSPF Version 2 July 1991
been sent by one of the router's active neighbors. If the receiving
interface is a multi-access network (either broadcast or non-broadcast)
the sender is identified by the IP source address found in the packet's
IP header. If the receiving interface is a point-to-point link or a
virtual link, the sender is identified by the Router ID (source router)
found in the packet's OSPF header. The data structure associated with
the receiving interface contains the list of active neighbors. Packets
not matching any active neighbor are discarded.
At this point all received protocol packets are associated with an
active neighbor. For the further input processing of specific packet
types, consult the sections listed in Table 11.
Type Packet name detailed section (receive)
________________________________________________________
1 Hello Section 10.5
2 Database description Section 10.6
3 Link state request Section 10.7
4 Link state update Section 13
5 Link state ack Section 13.7
Table 11: Sections describing packet reception.
9. The Interface Data Structure
An OSPF interface is the connection between a router and a network.
There is a single OSPF interface structure for each attached network;
each interface structure has at most one IP interface address (see
below). The support for multiple addresses on a single network is a
matter for future consideration.
An OSPF interface can be considered to belong to the area that contains
the attached network. All routing protocol packets originated by the
router over this interface are labelled with the interface's Area ID.
One or more router adjacencies may develop over an interface. A
router's link state advertisements reflect the state of its interfaces
and their associated adjacencies.
The following data items are associated with an interface. Note that a
number of these items are actually configuration for the attached
network; those items must be the same for all routers connected to the
network.
[Moy] [Page 42]RFC 1247 OSPF Version 2 July 1991
Type
The kind of network to which the interface attaches. Its value is
either broadcast, non-broadcast yet still multi-access, point-to-
point or virtual link.
State
The functional level of an interface. State determines whether or
not full adjacencies are allowed to form over the interface. State
is also reflected in the router's link state advertisements.
IP interface address
The IP address associated with the interface. This appears as the
IP source address in all routing protocol packets originated over
this interface. Interfaces to unnumbered point-to-point networks do
not have an associated IP address.
IP interface mask
This indicates the portion of the IP interface address that
identifies the attached network. This is often referred to as the
subnet mask. Masking the IP interface address with this value
yields the IP network number of the attached network.
Area ID
The Area ID to which the attached network belongs. All routing
protocol packets originating from the interface are labelled with
this Area ID.
HelloInterval
The length of time, in seconds, between the Hello packets that the
router sends on the interface. Advertised in Hello packets sent out
this interface.
RouterDeadInterval
The number of seconds before the router's neighbors will declare it
down, when they stop hearing the router's hellos. Advertised in
Hello packets sent out this interface.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface. Link state advertisements
contained in the update packet will have their age incremented by
this amount before transmission. This value should take into
account transmission and propagation delays; it must be greater than
zero.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
both attempt to become Designated Router, the one with the highest
[Moy] [Page 43]RFC 1247 OSPF Version 2 July 1991
Router Priority takes precedence. A router whose Router Priority is
set to 0 is ineligible to become Designated Router on the attached
network. Advertised in Hello packets sent out this interface.
Hello Timer
An interval timer that causes the interface to send a Hello packet.
This timer fires every HelloInterval seconds. Note that on non-
broadcast networks a separate Hello packet is sent to each qualified
neighbor.
Wait Timer
A single shot timer that causes the interface to exit the Waiting
state, and as a consequence select a Designated Router on the
network. The length of the timer is RouterDeadInterval seconds.
List of neighboring routers
The other routers attached to this network. On multi-access
networks, this list is formed by the Hello Protocol. Adjacencies
will be formed to some of these neighbors. The set of adjacent
neighbors can be determined by an examination of all of the
neighbors' states.
Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all multi-access networks by the
Hello Protocol. Two pieces of identification are kept for the
Designated Router: its Router ID and its interface IP address on the
network. The Designated Router advertises link state for the
network. The network link state advertisement is labelled with the
Designated Router's IP address. This item is initialized to 0,
which indicates the lack of a Designated Router.
Backup Designated Router
The Backup Designated Router is also selected on all multi-access
networks by the Hello Protocol. All routers on the attached network
become adjacent to both the Designated Router and the Backup
Designated Router. The Backup Designated Router becomes Designated
Router when the current Designated Router fails. Initialized to 0
indicating the lack of a Backup Designated Router.
Interface output cost(s)
The cost of sending a packet on the interface, expressed in the link
state metric. This is advertised as the link cost for this
interface in the router links advertisement. There may be a
separate cost for each IP Type of Service. The cost of an interface
must be greater than zero.
[Moy] [Page 44]RFC 1247 OSPF Version 2 July 1991
RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this interface. Also
used when retransmitting Database Description and Link State Request
Packets.
Authentication key
This configured data allows the authentication procedure to generate
and/or verify the authentication field in the OSPF header. The
authentication key can be configured on a per-interface basis. For
example, if the authentication type indicates simple password, the
authentication key would be a 64-bit password. This key would be
inserted directly into the OSPF header when originating routing
protocol packets, and there could be a separate password for each
network.
9.1 Interface states
The various states that router interface may attain is documented in
this section. The states are listed in order of progressing
functionality. For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved. The specification makes use of this
ordering by sometimes making references such as "those interfaces in
state greater than X".
Figure 11 shows the graph of interface state changes. The arcs of the
graph are labelled with the event causing the state change. These
events are documented in Section 9.2. The interface state machine is
described in more detail in Section 9.3.
Down
This is the initial interface state. In this state, the lower-level
protocols have indicated that the interface is unusable. No
protocol traffic at all will be sent or received on such a
interface. In this state, interface parameters should be set to
their initial values. All interface timers should be disabled, and
there should be no adjacencies associated with the interface.
Loopback
In this state, the router's interface to the network is looped back.
The interface may be looped back in hardware or software. The
interface will be unavailable for regular data traffic. However, it
may still be desirable to gain information on the quality of this
interface, either through sending ICMP pings to the interface or
through something like a bit error test. For this reason, IP
[Moy] [Page 45]RFC 1247 OSPF Version 2 July 1991
packets may still be addressed to an interface in Loopback state.
To facilitate this, such interfaces are advertised in router links
advertisements as single host routes, whose destination is the IP
interface address.[4]
Waiting
In this state, the router is trying to determine the identity of the
Backup Designated Router for the network. To do this, the router
monitors the Hellos it receives. The router is not allowed to elect
a Backup Designated Router nor Designated Router until it
transitions out of Waiting state. This prevents unnecessary changes
of (Backup) Designated Router.
Point-to-point
In this state, the interface is operational, and connects either to
a physical point-to-point network or to a virtual link. Upon
entering this state, the router attempts to form an adjacency with
the neighboring router. Hellos are sent to the neighbor every
HelloInterval seconds.
DR Other
The interface is to a multi-access network on which another router
has been selected to be the Designated Router. In this state, the
router itself has not been selected Backup Designated Router either.
The router forms adjacencies to both the Designated Router and the
Backup Designated Router (if they exist).
Backup
In this state, the router itself is the Backup Designated Router on
the attached network. It will be promoted to Designated Router when
the present Designated Router fails. The router establishes
adjacencies to all other routers attached to the network. The
Backup Designated Router performs slightly different functions
during the Flooding Procedure, as compared to the Designated Router
(see Section 13.3). See Section 7.4 for more details on the
functions performed by the Backup Designated Router.
DR In this state, this router itself is the Designated Router on the
attached network. Adjacencies are established to all other routers
attached to the network. The router must also originate a network
links advertisement for the network node. The advertisement will
contain links to all routers (including the Designated Router
itself) attached to the network. See Section 7.3 for more details
on the functions performed by the Designated Router.
[Moy] [Page 46]RFC 1247 OSPF Version 2 July 1991
9.2 Events causing interface state changes
State changes can be effected by a number of events. These events are
pictured as the labelled arcs in Figure 11. The label definitions are
listed below. For a detailed explanation of the effect of these events
on OSPF protocol operation, consult Section 9.3.
Interface Up
Lower-level protocols have indicated that the network interface is
operational. This enables the interface to transition out of Down
state. On virtual links, the interface operational indication is
actually a result of the shortest path calculation (see Section
16.7).
Wait Timer
The Wait timer has fired, indicating the end of the waiting period
that is required before electing a (Backup) Designated Router.
Backup seen
The router has detected the existence or non-existence of a Backup
Designated Router for the network. This is done in one of two ways.
First, a Hello Packet may be received from a neighbor claiming to be
itself the Backup Designated Router. Alternatively, a Hello Packet
may be received from a neighbor claiming to be itself the Designated
Router, and indicating that there is no Backup. In either case
there must be bidirectional communication with the neighbor, i.e.,
the router must also appear in the neighbor's Hello Packet. This
event signals an end to the Waiting state.
Neighbor Change
There has been a change in the set of bidirectional neighbors
associated with the interface. The (Backup) Designated Router needs
to be recalculated. The following neighbor changes lead to the
Neighbor Change event. For an explanation of neighbor states, see
Section 10.1.
o Bidirectional communication has been established to a neighbor.
In other words, the state of the neighbor has transitioned to
2-Way or higher.
o There is no longer bidirectional communication with a neighbor.
In other words, the state of the neighbor has transitioned to
Init or lower.
o One of the bidirectional neighbors is newly declaring itself as
either Designated Router or Backup Designated Router. This is
detected through examination of that neighbor's Hello Packets.
[Moy] [Page 47]RFC 1247 OSPF Version 2 July 1991
o One of the bidirectional neighbors is no longer declaring itself
as Designated Router, or is no longer declaring itself as Backup
Designated Router. This is again detected through examination
of that neighbor's Hello Packets.
o The advertised Router Priority for a bidirectional neighbor has
changed. This is again detected through examination of that
neighbor's Hello Packets.
Loop Ind
An indication has been received that the interface is now looped
back to itself. This indication can be received either from network
management or from the lower level protocols.
Unloop Ind
An indication has been received that the interface is no longer
looped back. As with the Loop Ind event, this indication can be
received either from network management or from the lower level
protocols.
Interface Down
Lower-level protocols indicate that this interface is no longer
functional. No matter what the current interface state is, the new
interface state will be Down.
9.3 The Interface state machine
A detailed description of the interface state changes follows. Each
state change is invoked by an event (Section 9.2). This event may
produce different effects, depending on the current state of the
interface. For this reason, the state machine below is organized by
current interface state and received event. Each entry in the state
machine describes the resulting new interface state and the required set
of additional actions.
When an interface's state changes, it may be necessary to originate a
new router links advertisement. See Section 12.4 for more details.
Some of the required actions below involve generating events for the
neighbor state machine. For example, when an interface becomes
inoperative, all neighbor connections associated with the interface must
be destroyed. For more information on the neighbor state machine, see
Section 10.3.
State(s): Down
[Moy] [Page 48]RFC 1247 OSPF Version 2 July 1991
Event: Interface Up
New state: Depends on action routine
Action: Start the interval Hello Timer, enabling the periodic
sending of Hello packets out the interface. If the attached
network is a physical point-to-point network or virtual
link, the interface state transitions to Point-to-Point.
Else, if the router is not eligible to become Designated
Router the interface state transitions to DR other.
Otherwise, the attached network is multi-access and the
router is eligible to become Designated Router. In this
case, in an attempt to discover the attached network's
Designated Router the interface state is set to Waiting and
the single shot Wait Timer is started. If in addition the
attached network is non-broadcast, examine the configured
list of neighbors for this interface and generate the
neighbor event Start for each neighbor that is also eligible
to become Designated Router.
State(s): Waiting
Event: Backup Seen
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated Router
and Designated Router, as shown in Section 9.4. As a result
of this calculation, the new state of the interface will be
either DR other, Backup or DR.
State(s): Waiting
Event: Wait Timer
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated Router
and Designated Router, as shown in Section 9.4. As a result
of this calculation, the new state of the interface will be
either DR other, Backup or DR.
State(s): DR Other, Backup or DR
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Event: Neighbor Change
New state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated Router
and Designated Router, as shown in Section 9.4. As a result
of this calculation, the new state of the interface will be
either DR other, Backup or DR.
State(s): Any State
Event: Interface Down
New state: Down
Action: All interface variables are reset, and interface timers
disabled. Also, all neighbor connections associated with
the interface are destroyed. This is done by generating the
event KillNbr on all associated neighbors (see Section
10.2).
State(s): Any State
Event: Loop Ind
New state: Loopback
Action: Since this interface is no longer connected to the attached
network the actions associated with the above Interface Down
event are executed.
State(s): Loopback
Event: Unloop Ind
New state: Down
Action: No actions are necessary. For example, the interface
variables have already been reset upon entering the Loopback
state. Note that reception of an Interface Up event is
necessary before the interface again becomes fully
functional.
[Moy] [Page 50]RFC 1247 OSPF Version 2 July 1991
9.4 Electing the Designated Router
This section describes the algorithm used for calculating a network's
Designated Router and Backup Designated Router. This algorithm is
invoked by the Interface state machine. The initial time a router runs
the election algorithm for a network, the network's Designated Router
and Backup Designated Router are initialized to 0.0.0.0. This indicates
the lack of both a Designated Router and a Backup Designated Router.
The Designated Router election algorithm proceeds as follows: Call the
router doing the calculation Router X. The list of neighbors attached
to the network and having established bidirectional communication with
Router X is examined. This list is precisely the collection of Router
X's neighbors (on this network) whose state is greater than or equal to
2-Way (see Section 10.1). Router X itself is also considered to be on
the list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0 are
ineligible to become Designated Router.) The following steps are then
executed, considering only those routers that remain on the list:
(1) Note the current values for the network's Designated Router and
Backup Designated Router. This is used later for comparison
purposes.
(2) Calculate the new Backup Designated Router for the network as
follows. Only those routers on the list that have not declared
themselves to be Designated Router are eligible to become Backup
Designated Router. If one or more of these routers have declared
themselves Backup Designated Router (i.e., they are currently
listing themselves as Backup Designated Router, but not as
Designated Router, in their Hello Packets) the one having highest
Router Priority is declared to be Backup Designated Router. In case
of a tie, the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router, choose
the router having highest Router Priority, (again excluding those
routers who have declared themselves Designated Router), and again
use the Router ID to break ties.
(3) Calculate the new Designated Router for the network as follows. If
one or more of the routers have declared themselves Designated
Router (i.e., they are currently listing themselves as Designated
Router in their Hello Packets) the one having highest Router
Priority is declared to be Designated Router. In case of a tie, the
one having the highest Router ID is chosen. If no routers have
declared themselves Designated Router, promote the new Backup
Designated Router to Designated Router.
[Moy] [Page 51]RFC 1247 OSPF Version 2 July 1991
(4) If Router X is now newly the Designated Router or newly the Backup
Designated Router, or is now no longer the Designated Router or no
longer the Backup Designated Router, repeat steps 2 and 3, and then
proceed to step 5. For example, if Router X is now the Designated
Router, when step 2 is repeated X will no longer be eligible for
Backup Designated Router election. Among other things, this will
ensure that no router will declare itself both Backup Designated
Router and Designated Router.[5]
(5) As a result of these calculations, the router itself may now be
Designated Router or Backup Designated Router. See Sections 7.3 and
7.4 for the additional duties this would entail. The router's
interface state should be set accordingly. If the router itself is
now Designated Router, the new interface state is DR. If the router
itself is now Backup Designated Router, the new interface state is
Backup. Otherwise, the new interface state is DR Other.
(6) If the attached network is non-broadcast, and the router itself has
just become either Designated Router or Backup Designated Router, it
must start sending hellos to those neighbors that are not eligible
to become Designated Router (see Section 9.5.1). This is done by
invoking the neighbor event Start for each neighbor having a Router
Priority of 0.
(7) If the above calculations have caused the identity of either the
Designated Router or Backup Designated Router to change, the set of
adjacencies associated with this interface will need to be modified.
Some adjacencies may need to be formed, and others may need to be
broken. To accomplish this, invoke the event AdjOK? on all
neighbors whose state is at least 2-Way. This will cause their
eligibility for adjacency to be reexamined (see Sections 10.3 and
10.4).
The reason behind the election algorithm's complexity is the desire for
an orderly transition from Backup Designated Router to Designated
Router, when the current Designated Router fails. This orderly
transition is ensured through the introduction of hysteresis: no new
Backup router can be chosen until the old Backup accepts its new
Designated Router responsibilities.
If Router X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a Designated Router
will be selected in the above procedure. Note also that if Router X is
the only attached router that is eligible to become Designated Router,
it will select itself as Designated Router and there will be no Backup
Designated Router for the network.
[Moy] [Page 52]RFC 1247 OSPF Version 2 July 1991
9.5 Sending Hello packets
Hello packets are sent out each functioning router interface. They are
used to discover and maintain neighbor relationships.[6] On multi-access
networks, hellos are also used to elect the Designated Router and Backup
Designated Router, and in that way determine what adjacencies should be
formed.
The format of a Hello packet is detailed in Section A.3.2. The Hello
Packet contains the router's Router Priority (used in choosing the
Designated Router), and the interval between Hello broadcasts
(HelloInterval). The Hello Packet also indicates how often a neighbor
must be heard from to remain active (RouterDeadInterval). Both
HelloInterval and RouterDeadInterval must be the same for all routers
attached to a common network. The Hello packet also contains the IP
address mask of the attached network (Network Mask). On unnumbered
point-to-point networks and on virtual links this field should be set to
0.
The Hello packet's Options field describes the router's optional OSPF
capabilities. There are currently two optional capabilities defined
(see Sections 4.5 and A.2). The T-bit of the Options field should be
set if the router is capable of calculating separate routes for each IP
TOS. The E-bit should be set if and only if the attached area is
capable of processing AS external advertisements (i.e., it is not a stub
area). If the E-bit is set incorrectly the neighboring routers will
refuse to accept the Hello Packet (see Section 10.5). The rest of the
Hello Packet's Options field should be set to zero.
In order to ensure two-way communication between adjacent routers, the
Hello packet contains the list of all routers from which hellos have
been seen recently. The Hello packet also contains the router's current
choice for Designated Router and Backup Designated Router. A value of 0
in these fields means that one has not yet been selected.
On broadcast networks and physical point-to-point networks, Hello
packets are sent every HelloInterval seconds to the IP multicast address
AllSPFRouters. On virtual links, Hello packets are sent as unicasts
(addressed directly to the other end of the virtual link) every
HelloInterval seconds. On non-broadcast networks, the sending of Hello
packets is more complicated. This will be covered in the next section.
9.5.1 Sending Hello packets on non-broadcast networks
Static configuration information is necessary in order for the Hello
Protocol to function on non-broadcast networks (see Section C.5). Every
attached router which is eligible to become Designated Router has a
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RFC 1247 OSPF Version 2 July 1991
configured list of all of its neighbors on the network. Each listed
neighbor is labelled with its Designated Router eligibility.
The interface state must be at least Waiting for any hellos to be sent.
Hellos are then sent directly (as unicasts) to some subset of a router's
neighbors. Sometimes an hello is sent periodically on a timer; at other
times it is sent as a response to a received hello. A router's hello-
sending behavior varies depending on whether the router itself is
eligible to become Designated Router.
If the router is eligible to become Designated Router, it must
periodically send hellos to all neighbors that are also eligible. In
addition, if the router is itself the Designated Router or Backup
Designated Router, it must also send periodic hellos to all other
neighbors. This means that any two eligible routers are always
exchanging hellos, which is necessary for the correct operation of the
Designated Router election algorithm. To minimize the number of hellos
sent, the number of eligible routers on a non-broadcast network should
be kept small.
If the router is not eligible to become Designated Router, it must
periodically send hellos to both the Designated Router and the Backup
Designated Router (if they exist). It must also send an hello in reply
to an hello received from any eligible neighbor (other than the current
Designated Router and Backup Designated Router). This is needed to
establish an initial bidirectional relationship with any potential
Designated Router.
When sending Hello packets periodically to any neighbor, the interval
between hellos is determined by the neighbor's state. If the neighbor
is in state Down, hellos are sent every PollInterval seconds.
Otherwise, hellos are sent every HelloInterval seconds.
10. The Neighbor Data Structure
An OSPF router converses with its neighboring routers. Each separate
conversation is described by a "neighbor data structure". Each
conversation is bound to a particular OSPF router interface, and is
identified either by the neighboring router's OSPF router ID or by its
Neighbor IP address (see below). Thus if the OSPF router and another
router have multiple attached networks in common, multiple conversations
ensue, each described by a unique neighbor data structure. Each
separate conversation is loosely referred to in the text as being a
separate "neighbor".
The neighbor data structure contains all information pertinent to the
forming or formed adjacency between the two neighbors. (However,
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RFC 1247 OSPF Version 2 July 1991
remember that not all neighbors become adjacent.) An adjacency can be
viewed as a highly developed conversation between two routers.
State
The functional level of the neighbor conversation. This is
described in more detail in Section 10.1.
Inactivity Timer
A single shot timer whose firing indicates that no Hello Packet has
been seen from this neighbor recently. The length of the timer is
RouterDeadInterval seconds.
Master/Slave
When the two neighbors are exchanging databases, they form a Master
Slave relationship. The Master sends the first Database Description
Packet, and is the only part that is allowed to retransmit. The
slave can only respond to the master's Database Description Packets.
The master/slave relationship is negotiated in state ExStart.
Sequence Number
A 32-bit number identifying individual Database Description packets.
When the neighbor state ExStart is entered, the sequence number
should be set to a value not previously seen by the neighboring
router. One possible scheme is to use the machine's time of day
counter. The sequence number is then incremented by the master with
each new Database Description packet sent. The slave's sequence
number indicates the last packet received from the master. Only one
packet is allowed outstanding at a time.
Neighbor ID
The OSPF Router ID of the neighboring router. The neighbor ID is
learned when Hello packets are received from the neighbor, or is
configured if this is a virtual adjacency (see Section C.4).
Neighbor priority
The Router Priority of the neighboring router. Contained in the
neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.
Neighbor IP address
The IP address of the neighboring router's interface to the attached
network. Used as the Destination IP address when protocol packets
are sent as unicasts along this adjacency. Also used in router
links advertisements as the Link ID for the attached network if the
neighboring router is selected to be Designated Router (see Section
12.4.1). The neighbor IP address is learned when Hello packets are
received from the neighbor. For virtual links, the neighbor IP
[Moy] [Page 55]RFC 1247 OSPF Version 2 July 1991
address is learned during the routing table build process (see
Section 15).
Neighbor Options
The optional OSPF capabilities supported by the neighbor. Learned
during the Database Exchange process (see Section 10.6). The
neighbor's optional OSPF capabilities are also listed in its Hello
packets. This enables received Hellos to be rejected (i.e.,
neighbor relationships will not even start to form) if there is a
mismatch in certain crucial OSPF capabilities (see Section 10.5).
The optional OSPF capabilities are documented in Section 4.5.
Neighbor's Designated Router
The neighbor's idea of the Designated Router. If this is the
neighbor itself, this is important in the local calculation of the
Designated Router. Defined only on multi-access networks.
Neighbor's Backup Designated Router
The neighbor's idea of the Backup Designated Router. If this is the
neighbor itself, this is important in the local calculation of the
Backup Designated Router. Defined only on multi-access networks.
The next set of variables are lists of link state advertisements. These
lists describe subsets of the area topological database. There can be
five distinct types of link state advertisements in an area topological
database: router links, network links, and type 3 and 4 summary links
(all stored in the area data structure), and AS external links (stored
in the global data structure).
Link state retransmission list
The list of link state advertisements that have been flooded but not
acknowledged on this adjacency. These will be retransmitted at
intervals until they are acknowledged, or until the adjacency is
destroyed.
Database summary list
The complete list of link state advertisements that make up the area
topological database, at the moment the neighbor goes into Database
Exchange state. This list is sent to the neighbor in Database
Description packets.
Link state request list
The list of link state advertisements that need to be received from
this neighbor in order to synchronize the two neighbors' topological
databases. This list is created as Database Description packets are
received, and is then sent to the neighbor in Link State Request
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packets. The list is depleted as appropriate Link State Update
packets are received.
10.1 Neighbor states
The state of a neighbor (really, the state of a conversation being held
with a neighboring router) is documented in the following sections. The
states are listed in order of progressing functionality. For example,
the inoperative state is listed first, followed by a list of
intermediate states before the final, fully functional state is
achieved. The specification makes use of this ordering by sometimes
making references such as "those neighbors/adjacencies in state greater
than X". Figures 12 and 13 show the graph of neighbor state changes.
The arcs of the graphs are labelled with the event causing the state
change. The neighbor events are documented in Section 10.2.
The graph in Figure 12 show the state changes effected by the Hello
Protocol. The Hello Protocol is responsible for neighbor acquisition
and maintenance, and for ensuring two way communication between
neighbors.
The graph in Figure 13 shows the forming of an adjacency. Not every two
neighboring routers become adjacent (see Section 10.4). The adjacency
starts to form when the neighbor is in state ExStart. After the two
routers discover their master/slave status, the state transitions to
Exchange. At this point the neighbor starts to be used in the flooding
procedure, and the two neighboring routers begin synchronizing their
databases. When this synchronization is finished, the neighbor is in
state Full and we say that the two routers are fully adjacent. At this
point the adjacency is listed in link state advertisements.
For a more detailed description of neighbor state changes, together with
the additional actions involved in each change, see Section 10.3.
_____________________________________________________
(Figures not included in text version.)
Figure 12: Neighbor state changes (Hello Protocol)
Figure 13: Neighbor state changes (Database Exchange)
_____________________________________________________
[Moy] [Page 57]RFC 1247 OSPF Version 2 July 1991
Down
This is the initial state of a neighbor conversation. It indicates
that there has been no recent information received from the
neighbor. On non-broadcast networks, Hello packets may still be
sent to "Down" neighbors, although at a reduced frequency (see
Section 9.5.1).
Attempt
This state is only valid for neighbors attached to non-broadcast
networks. It indicates that no recent information has been received
from the neighbor, but that a more concerted effort should be made
to contact the neighbor. This is done by sending the neighbor Hello
packets at intervals of HelloInterval (see Section 9.5.1).
Init
In this state, an Hello packet has recently been seen from the
neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). All neighbors in this state
(or higher) are listed in the Hello packets sent from the associated
interface.
2-Way
In this state, communication between the two routers is
bidirectional. This has been assured by the operation of the Hello
Protocol. This is the most advanced state short of beginning
adjacency establishment. The (Backup) Designated Router is selected
from the set of neighbors in state 2-Way or greater.
ExStart
This is the first step in creating an adjacency between the two
neighboring routers. The goal of this step is to decide which
router is the master, and to decide upon the initial sequence
number. Neighbor conversations in this state or greater are called
adjacencies.
Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the neighbor.
Each Database Description Packet has a sequence number, and is
explicitly acknowledged. Only one Database Description Packet is
allowed outstanding at any one time. In this state, Link State
Request Packets may also be sent asking for the neighbor's more
recent advertisements. All adjacencies in Exchange state or greater
are used by the flooding procedure. In fact, these adjacencies are
fully capable of transmitting and receiving all types of OSPF
routing protocol packets.
[Moy] [Page 58]RFC 1247 OSPF Version 2 July 1991
Loading
In this state, Link State Request packets are sent to the neighbor
asking for the more recent advertisements that have been discovered
(but not yet received) in the Exchange state.
Full
In this state, the neighboring routers are fully adjacent. These
adjacencies will now appear in router links and network links
advertisements.
10.2 Events causing neighbor state changes
State changes can be effected by a number of events. These events are
shown in the labels of the arcs in Figures 12 and 13. The label
definitions are as follows:
Hello Received
A Hello packet has been received from a neighbor.
Start
This is an indication that Hello Packets should now be sent to the
neighbor at intervals of HelloInterval seconds. This event is
generated only for neighbors associated with non-broadcast networks.
2-Way Received
Bidirectional communication has been realized between the two
neighboring routers. This is indicated by this router seeing itself
in the other's Hello packet.
NegotiationDone
The Master/Slave relationship has been negotiated, and sequence
numbers have been exchanged. This signals the start of the
sending/receiving of Database Description packets. For more
information on the generation of this event, consult Section 10.8.
Exchange Done
Both routers have successfully transmitted a full sequence of
Database Description packets. Each router now knows what parts of
its link state database are out of date. For more information on
the generation of this event, consult Section 10.8.
BadLSReq
A Link State Request has been received for a link state
advertisement not contained in the database. This indicates an
error in the synchronization process.
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Loading Done
Link State Updates have been received for all out-of-date portions
of the database. This is indicated by the Link state request list
becoming empty after the Database Description Process has completed.
AdjOK?
A decision must be made (again) as to whether an adjacency should be
established/maintained with the neighbor. This event will start
some adjacencies forming, and destroy others.
The following events cause well developed neighbors to revert to lesser
states. Unlike the above events, these events may occur when the
neighbor conversation is in any of a number of states.
Seq Number Mismatch
A Database Description packet has been received that either a) has
an unexpected sequence number, b) unexpectedly has the Init bit set
or c) has an Options field differing from the last Options field
received in a Database Description packet. Any of these conditions
indicate that some error has occurred during adjacency
establishment.
1-Way
An Hello packet has been received from the neighbor, in which this
router is not mentioned. This indicates that communication with the
neighbor is not bidirectional.
KillNbr
This is an indication that all communication with the
neighbor is now impossible, forcing the neighbor to revert
to Down state.
Inactivity Timer
The inactivity Timer has fired. This means that no Hello packets
have been seen recently from the neighbor. The neighbor reverts to
Down state.
LLDown
This is an indication from the lower level protocols that the
neighbor is now unreachable. For example, on an X.25 network this
could be indicated by an X.25 clear indication with appropriate
cause and diagnostic fields. This event forces the neighbor into
Down state.
[Moy] [Page 60]RFC 1247 OSPF Version 2 July 1991
10.3 The Neighbor state machine
A detailed description of the neighbor state changes follows. Each
state change is invoked by an event (Section 10.2). This event may
produce different effects, depending on the current state of the
neighbor. For this reason, the state machine below is organized by
current neighbor state and received event. Each entry in the state
machine describes the resulting new neighbor state and the required set
of additional actions.
When an neighbor's state changes, it may be necessary to rerun the
Designated Router election algorithm. This is determined by whether the
interface Neighbor Change event is generated (see Section 9.2). Also,
if the Interface is in DR state (the router is itself Designated
Router), changes in neighbor state may cause a new network links
advertisement to be originated (see Section 12.4).
When the neighbor state machine needs to invoke the interface state
machine, it should be done as a scheduled task (see Section 4.4). This
simplifies things, by ensuring that neither state machine will be
executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an hello to the neighbor (this neighbor is always
associated with a non-broadcast network) and start the
inactivity timer for the neighbor. The timer's later firing
would indicate that communication with the neighbor was not
attained.
State(s): Attempt
Event: Hello Received
New state: Init
Action: Restart the inactivity timer for the neighbor, since the
neighbor has now been heard from.
State(s): Down
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Event: Hello Received
New state: Init
Action: Start the inactivity timer for the neighbor. The timer's
later firing would indicate that the neighbor is dead.
State(s): Init or greater
Event: Hello Received
New state: No state change.
Action: Restart the inactivity timer for the neighbor, since the
neighbor has again been heard from.
State(s): Init
Event: 2-Way Received
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established with
the neighbor (see Section 10.4). If not, the new neighbor
state is 2-Way.
Otherwise (an adjacency should be established) the neighbor
state transitions to ExStart. Upon entering this state, the
router increments the sequence number for this neighbor. If
this is the first time that an adjacency has been attempted,
the sequence number should be assigned some unique value
(like the time of day clock). It then declares itself
master (sets the master/slave bit to master), and starts
sending Database Description Packets, with the initialize
(I), more (M) and master (MS) bits set. This Database
Description Packet should be otherwise empty. This Database
Description Packet should be retransmitted at intervals of
RxmtInterval until the next state is entered (see Section
10.8).
State(s): ExStart
Event: NegDone
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New state: Exchange
Action: The router must list the contents of its entire area link
state database in the neighbor Database summary list. The
area link state database consists of the router links,
network links and summary links contained in the area
structure, along with the AS external links contained in the
global structure. AS external link advertisements are
omitted from a virtual neighbor's Database summary list. AS
external advertisements are omitted from the Database
summary list if the area has been configured as a stub (see
Section 3.6). Advertisements whose age is equal to MaxAge
are instead added to the neighbor's Link state
retransmission list. A summary of the Database summary list
will be sent to the neighbor in Database Description
packets. Each Database Description Packet has a sequence
number, and is explicitly acknowledged. Only one Database
Description Packet is allowed outstanding at any one time.
For more detail on the sending and receiving of Database
Description packets, see Sections 10.8 and 10.6.
State(s): Exchange
Event: Exchange Done
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty, the new
neighbor state is Full. No other action is required. This
is an adjacency's final state.
Otherwise, the new neighbor state is Loading. Start (or
continue) sending Link State Request packets to the neighbor
(see Section 10.9). These are requests for the neighbor's
more recent advertisements (which were discovered but not
yet received in the Exchange state). These advertisements
are listed in the Link state request list associated with
the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
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Action: No action required. This is an adjacency's final state.
State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with the
neighboring router (see Section 10.4). If not, the neighbor
state remains at 2-Way. Otherwise, transition the neighbor
state to ExStart and perform the actions associated with the
above state machine entry for state Init and event 2-Way
Received.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should still be
adjacent. If yes, there is no state change and no further
action is necessary.
Otherwise, the (possibly partially formed) adjacency must be
destroyed. The neighbor state transitions to 2-Way. The
Link state retransmission list, Database summary list and
Link state request list are cleared of link state
advertisements.
State(s): Exchange or greater
Event: Seq Number Mismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn down, and
then an attempt is made at reestablishment. The neighbor
state first transitions to ExStart. The Link state
retransmission list, Database summary list and Link state
request list are cleared of link state advertisements. Then
the router increments the sequence number for this neighbor,
declares itself master (sets the master/slave bit to
master), and starts sending Database Description Packets,
[Moy] [Page 64]RFC 1247 OSPF Version 2 July 1991
with the initialize (I), more (M) and master (MS) bits set.
This Database Description Packet should be otherwise empty
(see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as for the
neighbor event SeqNumberMismatch. The (possibly partially
formed) adjacency is torn down, and then an attempt is made
at reestablishment. For more information, see the neighbor
state machine entry that is invoked when event
SeqNumberMismatch is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
State(s): Any state
Event: Inactivity Timer
New state: Down
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Action: The Link state retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements.
State(s): 2-Way or greater
Event: 1-Way Received
New state: Init
Action: The Link state retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements.
State(s): 2-Way or greater
Event: 2-Way received
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-Way received
New state: No state change.
Action: No action required.
10.4 Whether to become adjacent
Adjacencies are established with some subset of the router's neighbors.
Routers connected by point-to-point networks and virtual links always
become adjacent. On multi-access networks, all routers become adjacent
to both the Designated Router and the Backup Designated Router.
The adjacency-forming decision occurs in two places in the neighbor
state machine. First, when bidirectional communication is initially
established with the neighbor, and secondly, when the identity of the
attached network's (Backup) Designated Router changes. If the decision
is made to not attempt an adjacency, the state of the neighbor
communication stops at 2-Way.
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An adjacency should be established with a (bidirectional) neighbor when
at least one of the following conditions holds:
o The underlying network type is point-to-point
o The underlying network type is virtual link
o The router itself is the Designated Router
o The router itself is the Backup Designated Router
o The neighboring router is the Designated Router
o The neighboring router is the Backup Designated Router
10.5 Receiving Hello packets
This section explains the detailed processing of a received Hello
packet. (See Section A.3.2 for the format of Hello packets.) The
generic input processing of OSPF packets will have checked the validity
of the IP header and the OSPF packet header. Next, the values of the
Network Mask, HelloInt, and DeadInt fields in the received Hello packet
must be checked against the values configured for the receiving
interface. Any mismatch causes processing to stop and the packet to be
dropped. In other words, the above fields are really describing the
attached network's configuration. Note that the value of the Network
Mask field should not be checked in Hellos received on unnumbered serial
lines or on virtual links.
The receiving interface attaches to a single OSPF area (this could be
the backbone). The setting of the E-bit found in the Hello Packet's
option field must match this area's external routing capability. If AS
external advertisements are not flooded into/throughout the area (i.e,
the area is a "stub") the E-bit must be clear in received hellos,
otherwise the E-bit must be set. A mismatch causes processing to stop
and the packet to be dropped. The setting of the rest of the bits in
the Hello Packet's option field should be ignored.
At this point, an attempt is made to match the source of the Hello
Packet to one of the receiving interface's neighbors. If the receiving
interface is a multi-access network (either broadcast or non-broadcast)
the source is identified by the IP source address found in the Hello's
IP header. If the receiving interface is a point-to-point link or a
virtual link, the source is identified by the Router ID found in the
Hello's OSPF packet header. The interface's current list of neighbors
is contained in the interface's data structure. If a matching neighbor
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structure cannot be found, (i.e., this is the first time the neighbor
has been detected), one is created. The initial state of a newly
created neighbor is set to Down.
When receiving an Hello Packet from a neighbor on a multi-access network
(broadcast or non-broadcast), set the neighbor structure's Neighbor ID
equal to the Router ID found in the packet's OSPF header. When
receiving an Hello on a point-to-point network (but not on a virtual
link) set the neighbor structure's Neighbor IP address to the packet's
IP source address.
Now the rest of the Hello Packet is examined, generating events to be
given to the neighbor and interface state machines. These state
machines are specified either to be executed or scheduled (see Section
4.4). For example, by specifying below that the neighbor state machine
be executed in line, several neighbor state transitions may be effected
by a single received Hello:
o Each Hello Packet causes the neighbor state machine to be executed
with the event Hello Received.
o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the neighbor
state machine should be executed with the event 2-Way Received.
Otherwise, the neighbor state machine should be executed with the
event 1-Way Received, and the processing of the packet stops.
o Next, the Hello packet's Router Priority field is examined. If this
field is different than the one previously received from the
neighbor, the receiving interface's state machine is scheduled with
the event NeighborChange. In any case, the Router Priority field in
the neighbor data structure should be set accordingly.
o Next the Designated Router field in the Hello Packet is examined.
If the neighbor is both declaring itself to be Designated Router
(Designated Router field = neighbor IP address) and the Backup
Designated Router field in the packet is equal to 0.0.0.0 and the
receiving interface is in state Waiting, the receiving interface's
state machine is scheduled with the event BackupSeen. Otherwise, if
the neighbor is declaring itself to be Designated Router and it had
not previously, or the neighbor is not declaring itself Designated
Router where it had previously, the receiving interface's state
machine is scheduled with the event NeighborChange. In any case,
the Designated Router item in the neighbor structure is set
accordingly.
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o Finally, the Backup Designated Router field in the Hello Packet is
examined. If the neighbor is declaring itself to be Backup
Designated Router (Backup Designated Router field = neighbor IP
address) and the receiving interface is in state Waiting, the
receiving interface's state machine is scheduled with the event
BackupSeen. Otherwise, if the neighbor is declaring itself to be
Backup Designated Router and it had not previously, or the neighbor
is not declaring itself Backup Designated Router where it had
previously, the receiving interface's state machine is scheduled
with the event NeighborChange. In any case, the Backup Designated
Router item in the neighbor structure is set accordingly.
10.6 Receiving Database Description Packets
This section explains the detailed processing of a received Database
Description packet. The incoming Database Description Packet has
already been associated with a neighbor and receiving interface by the
generic input packet processing (Section 8.2). The further processing
of the Database Description Packet depends on the neighbor state. If
the neighbor's state is Down or Attempt the packet should be ignored.
Otherwise, if the state is:
Init
The neighbor state machine should be executed with the event 2-Way
Received. This causes an immediate state change to either state 2-
Way or state Exstart. The processing of the current packet should
then continue in this new state.
2-Way
The packet should be ignored. Database description packets are used
only for the purpose of bringing up adjacencies.[7]
ExStart
If the received packet matches one of the following cases, then the
neighbor state machine should be executed with the event
NegotiationDone (causing the state to transition to Exchange), the
packet's Options field should be recorded in the neighbor
structure's Neighbor Options field and the packet should be accepted
as next in sequence and processed further (see below). Otherwise,
the packet should be ignored.
o The initialize(I), more (M) and master(MS) bits are set, the
contents of the packet are empty, and the neighbor's Router ID
is larger than the router's own. In this case the router is now
Slave. Set the master/slave bit to slave, and set the sequence
number to that specified by the master.
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o The initialize(I) and master(MS) bits are off, the packet's
sequence number equals the router's own sequence number
(indicating acknowledgment) and the neighbor's Router ID is
smaller than the router's own. In this case the router is
Master.
Exchange
If the state of the MS-bit is inconsistent with the master/slave
state of the connection, generate the neighbor event Seq Number
Mismatch and stop processing the packet. Otherwise:
o If the initialize(I) bit is set, generate the neighbor event Seq
Number Mismatch and stop processing the packet.
o If the packet's Options field indicates a different set of
optional OSPF capabilities than were previously received from
the neighbor (recorded in the Neighbor Options field of the
neighbor structure), generate the neighbor event Seq Number
Mismatch and stop processing the packet.
o If the router is master, and the packet's sequence number equals
the router's own sequence number (this packet is the next in
sequence) the packet should be accepted and its contents
processed (below).
o If the router is master, and the packet's sequence number is one
less than the router's sequence number, the packet is a
duplicate. Duplicates should be discarded by the master.
o If the router is slave, and the packet's sequence number is one
more than the router's own sequence number (this packet is the
next in sequence) the packet should be accepted and its contents
processed (below).
o If the router is slave, and the packet's sequence number is
equal to the router's sequence number, the packet is a
duplicate. The slave must respond to duplicates by repeating
the last Database Description packet that it sent.
o Else, generate the neighbor event Seq Number Mismatch and stop
processing the packet.
Loading or Full
In this state, the router has sent and received an entire sequence
of Database Descriptions. The only packets received should be
duplicates (see above). In particular, the packet's Options field
should match the set of optional OSPF capabilities previously
indicated by the neighbor (stored in the neighbor structure's
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neighbor Options field). Any other packets received, including the
reception of a packet with the Initialize(I) bit set, should
generate the neighbor event Seq Number Mismatch.[8] Duplicates
should be discarded by the master. The slave must respond to
duplicates by repeating the last Database Description packet that it
sent.
When the router accepts a received Database Description Packet as the
next in sequence the packet contents are processed as follows. For each
link state advertisement listed, the advertisement's LS type is checked
for validity. If the LS type is unknown (e.g., not one of the LS types
1-5 defined by this specification), or if this is a AS external
advertisement (LS type = 5) and the neighbor is associated with a stub
area, generate the neighbor event Seq Number Mismatch and stop
processing the packet. Otherwise, the router looks up the advertisement
in its database to see whether it also has an instance of the link state
advertisement. If it does not, or if the database copy is less recent
(see Section 13.1), the link state advertisement is put on the Link
state request list so that it can be requested (immediately or at some
later time) in Link State Request Packets.
When the router accepts a received Database Description Packet as the
next in sequence, it also performs the following actions, depending on
whether it is master or slave:
Master
Increments the sequence number. If the router has already sent its
entire sequence of Database Descriptions, and the just accepted
packet has the more bit (M) set to 0, the neighbor event Exchange
Done is generated. Otherwise, it should send a new Database
Description to the slave.
Slave
Sets the sequence number to the sequence number appearing in the
received packet. The slave must send a Database Description in
reply. If the received packet has the more bit (M) set to 0, and
the packet to be sent by the slave will have the M-bit set to 0
also, the neighbor event Exchange Done is generated. Note that the
slave always generates this event before the master.
10.7 Receiving Link State Request Packets
This section explains the detailed processing of received Link State
Request packets. Received Link State Request Packets specify a list of
link state advertisements that the neighbor wishes to receive. Link
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state Request Packets should be accepted when the neighbor is in states
Exchange, Loading, or Full. In all other states Link State Request
Packets should be ignored.
Each link state advertisement specified in the Link State Request packet
should be located in the router's database, and copied into Link State
Update packets for transmission to the neighbor. These link state
advertisements should NOT be placed on the Link state retransmission
list for the neighbor. If a link state advertisement cannot be found in
the database, something has gone wrong with the synchronization
procedure, and neighbor event BadLSReq should be generated.
10.8 Sending Database Description Packets
This section describes how Database Description Packets are sent to a
neighbor. The router's optional OSPF capabilities (see Section 4.5) are
transmitted to the neighbor in the Options field of the Database
Description packet. The router should maintain the same set of optional
capabilities throughout the Database Exchange and flooding procedures.
If for some reason the router's optional capabilities change, the
Database Exchange procedure should be restarted by reverting to neighbor
state ExStart. There are currently two optional capabilities defined.
The T-bit should be set if and only if the router is capable of
calculating separate routes for each IP TOS. The E-bit should be set if
and only if the attached network belongs to a non-stub area. The rest
of the Options field should be set to zero.
The sending of Database Description packets depends on the neighbor's
state. In state ExStart the router sends empty Database Description
packets, with the initialize (I), more (M) and master (MS) bits set.
These packets are retransmitted every RxmtInterval seconds.
In state Exchange the Database Description Packets actually contain
summaries of the link state information contained in the router's
database. Each link state advertisement in the area's topological
database (at the time the neighbor transitions into Exchange state) is
listed in the neighbor Database summary list. When a new Database
Description Packet is to be sent, the packet's sequence number is
incremented, and the (new) top of the Database summary list is described
by the packet. Items are removed from the Database summary list when
the previous packet is acknowledged.
In state Exchange, the determination of when to send a packet depends on
whether the router is master or slave:
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Master
Packets are sent when either a) the slave acknowledges the previous
packet by echoing the sequence number or b) RxmtInterval seconds
elapse without an acknowledgment, in which case the previous packet
is retransmitted.
Slave
Packets are sent only in response to packets received from the
master. If the packet received from the master is new, a new packet
is sent, otherwise the previous packet is resent.
In states Loading and Full the slave must resend its last packet in
response to duplicate packets received from the master. For this reason
the slave must wait RouterDeadInterval seconds before freeing the last
packet. Reception of a packet from the master after this interval will
generate a Seq Number Mismatch neighbor event.
10.9 Sending Link State Request Packets
In neighbor states Exchange or Loading, the Link state request list
contains a list of those link state advertisements that need to be
obtained from the neighbor. To request these advertisements, a router
sends the neighbor the beginning of the Link state request list,
packaged in a Link State Request packet.
When the neighbor responds to these requests with the proper Link State
Update packet(s), the Link state request list is truncated and a new
Link State Request packet is sent. This process continues until the
link state request list becomes empty. Unsatisfied Link State Requests
are retransmitted at intervals of RxmtInterval. There should be at most
one Link State Request packet outstanding at any one time.
When the Link state request list becomes empty, and the neighbor state
is Loading (i.e., a complete sequence of Database Description packets
has been received from the neighbor), the Loading Done neighbor event is
generated.
10.10 An Example
Figure 14 shows an example of an adjacency forming. Routers RT1 and RT2
are both connected to a broadcast network. It is assumed that RT2 is
the Designated Router for the network, and that RT2 has a higher Router
ID that router RT1.
The neighbor state changes realized by each router are listed on the
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sides of the figure.
At the beginning of Figure 14, router RT1's interface to the network
becomes operational. It begins sending hellos, although it doesn't know
the identity of the Designated Router or of any other neighboring
routers. Router RT2 hears this hello (moving the neighbor to Init
state), and in its next hello indicates that it is itself the Designated
Router and that it has heard hellos from RT1. This in turn causes RT1
to go to state ExStart, as it starts to bring up the adjacency.
RT1 begins by asserting itself as the master. When it sees that RT2 is
indeed the master (because of RT2's higher Router ID), RT1 transitions
to slave state and adopts its neighbor's sequence number. Database
Description packets are then exchanged, with polls coming from the
master (RT2) and responses from the slave (RT1). This sequence of
Database Description Packets ends when both the poll and associated
response has the M-bit off.
In this example, it is assumed that RT2 has a completely up to date
database. In that case, RT2 goes immediately into Full state. RT1 will
go into Full state after updating the necessary parts of its database.
This is done by sending Link State Request Packets, and receiving Link
State Update Packets in response. Note that, while RT1 has waited until
a complete set of Database Description Packets has been received (from
RT2) before sending any Link State Request Packets, this need not be the
case. RT1 could have interleaved the sending of Link State Request
Packets with the reception of Database Description Packets.
11. The Routing Table Structure
The routing table data structure contains all the information necessary
to forward an IP data packet toward its destination. Each routing table
entry describes the collection of best paths to a particular
destination. When forwarding an IP data packet, the routing table entry
providing the best match for the packet's IP destination is located.
________________________________________
(Figure not included in text version.)
Figure 14: An adjacency bring-up example
________________________________________
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The matching routing table entry then provides the next hop towards the
packet's destination. OSPF also provides for the existence of a default
route (Destination ID = DefaultDestination). When the default route
exists, it matches all IP destinations (although any other matching
entry is a better match). Finding the routing table entry that best
matches an IP destination is further described in Section 11.1.
There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.
The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.
Destination Type
The destination can be one of three types. Only the first type,
Network, is actually used when forwarding IP data traffic. The
other destinations are used solely as intermediate steps in the
routing table build process.
Network
A range of IP addresses, to which IP data traffic may be
forwarded. This includes IP networks (class A, B, or C), IP
subnets, and single IP hosts. The default route also falls in
this category.
Area border router
Routers that are connected to multiple OSPF areas. Such routers
originate summary link advertisements. These routing table
entries are used when calculating the inter-area routes (see
Section 16.2). These routing table entries may also be
associated with configured virtual links.
AS boundary router
Routers that originate AS external link advertisements. These
routing table entries are used when calculating the AS external
routes (see Section 16.4).
Destination ID
The destination's identifier or name. This depends on the
destination's type. For networks, the identifier is their
associated IP address. For all other types, the identifier is the
OSPF Router ID.[9]
Address Mask
Only defined for networks. The network's IP address together with
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its address mask defines a range of IP addresses. For IP subnets,
the address mask is referred to as the subnet mask. For host
routes, the mask is "all ones" (0xffffffff).
Optional Capabilities
When the destination is a router (either an area border router or an
AS boundary router) this field indicates the optional OSPF
capabilities supported by the destination router. The two optional
capabilities currently defined by this specification are the ability
to route based on IP TOS and the ability to process AS external
advertisements. For a further discussion of OSPF's optional
capabilities, see Section 4.5.
The set of paths to use for a destination may vary based on IP Type of
Service and the OSPF area to which the paths belong. This means that
there may be multiple routing table entries for the same destination,
depending on the values of the next two fields.
Type of Service
There can be a separate set of routes for each IP Type of Service.
The encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
Area
This field indicates the area whose link state information has led
to the routing table entry's collection of paths. This is called
the entry's associated area. For sets of AS external paths, this
field is not defined. For destinations of type "area border
router", there may be separate sets of paths (and therefore separate
routing table entries) associated with each of several areas. This
will happen when two area border routers share multiple areas in
common. For all other destination types, only the set of paths
associated with the best area (the one providing the shortest route)
is kept.
The rest of the routing table entry describes the set of paths to the
destination. The following fields pertain to the set of paths as a
whole. In other words, each one of the paths contained in a routing
table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to the
destination, listed here in order of preference: intra-area, inter-
area, type 1 external or type 2 external. Intra-area paths indicate
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destinations belonging to one of the router's attached areas.
Inter-area paths are paths to destinations in other OSPF areas.
These are discovered through the examination of received summary
link advertisements. AS external paths are paths to destinations
external to the AS. These are detected through the examination of
received AS external link advertisements.
Cost
The link state cost of the path to the destination. For all paths
except type 2 external paths this describes the entire path's cost.
For Type 2 external paths, this field describes the cost of the
portion of the path internal to the AS. This cost is calculated as
the sum of the costs of the path's constituent links.
Type 2 cost
Only valid for type 2 external paths. For these paths, this field
indicates the cost of the path's external portion. This cost has
been advertised by an AS boundary router, and is the most
significant part of the total path cost. For example, an external
type 2 path with type 2 cost of 5 is always preferred over a path
with type 2 cost of 10, regardless of the cost of the two paths'
internal components.
Link State Origin
Valid only for intra-area paths, this field indicates the link state
advertisement (router links or network links) that directly
references the destination. For example, if the destination is a
transit network, this is the transit network's network links
advertisement. If the destination is a stub network, this is the
router links advertisement for the attached router. The
advertisement is discovered during the shortest-path tree
calculation (see Section 16.1). Multiple advertisements may
reference the destination, however a tie-breaking scheme always
reduces the choice to a single advertisement.
This field is for informational purposes only. The advertisement
could be used as a root for an SPF calculation when building a
reverse path forwarding tree. This is beyond the scope of this
specification.
When multiple paths of equal path-type and cost exist to a destination
(called elsewhere "equal-cost" paths), they are stored in a single
routing table entry. Each one of the "equal-cost" paths is
distinguished by the following fields:
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Next hop
The outgoing router interface to use when forwarding traffic to the
destination. On multi-access networks, the next hop also includes
the IP address of the next router (if any) in the path towards the
destination. This next router will always be one of the adjacent
neighbors.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary link
or AS external link that led to this path.
11.1 Routing table lookup
When an IP data packet is received, an OSPF router finds the routing
table entry that best matches the packet's destination. This routing
table entry then provides the outgoing interface and next hop router to
use in forwarding the packet. This section describes the process of
finding the best matching routing table entry. The process consists of a
number of steps, wherein the collection of routing table entries is
progressively pruned. In the end, the single routing table entry
remaining is the called best match.
Note that the steps described below may fail to produce a best match
routing table entry (i.e., all existing routing table entries are pruned
for some reason or another). In this case, the packet's IP destination
is considered unreachable. Instead of being forwarded, the packet should
be dropped and an ICMP destination unreachable message should be
returned to the packet's source.
(1) Select the complete set of "matching" routing table entries from the
routing table. Each routing table entry describes a (set of)
path(s) to a range of IP addresses. If the data packet's IP
destination falls into an entry's range of IP addresses, the routing
table entry is called a match. (It is quite likely that multiple
entries will match the data packet. For example, a default route
will match all packets.)
(2) Suppose that the packet's IP destination falls into one of the
router's configured area address ranges (see Section 3.5), and that
the particular area address range is active. This means that there
are one or more reachable (by intra-area paths) networks contained
in the area address range. The packet's IP destination is then
required to belong to one of these constituent networks. For this
reason, only matching routing table entries with path-type of
intra-area are considered (all others are pruned). If no such
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matching entries exist, the destination is unreachable (see above).
Otherwise, skip to step 4.
(3) Reduce the set of matching entries to those having the most
preferential path-type (see Section 11). OSPF has a four level
hierarchy of paths. Intra-area paths are the most preferred,
followed in order by inter-area, Type 1 external and Type 2 external
paths.
(4) Select the remaining routing table entry that provides the longest
(most specific) match. Another way of saying this is to choose the
remaining entry that specifies the narrowest range of IP
addresses.[10] For example, the entry for the address/mask pair of
(128.185.1.0, 0xffffff00) is more specific than an entry for the
pair (128.185.0.0, 0xffff0000). The default route is the least
specific match, since it matches all destinations.
(5) At this point, there may still be multiple routing table entries
remaining. Each routing entry will specify the same range of IP
addresses, but a different IP Type of Service. Select the routing
table entry whose TOS value matches the TOS found in the packet
header. If there is no routing table entry for this TOS, select the
routing table entry for TOS 0. In other words, packets requesting
TOS X are routed along the TOS 0 path if a TOS X path does not
exist.
11.2 Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF areas have
been configured. A single metric is shown per outbound interface,
indicating that routes will not vary based on TOS. The calculation
router RT6's routing table proceeds as described in Section 2.1. The
resulting routing table is shown in Table 12. Destination types are
abbreviated: Network as "N", area border router as "BR" and AS boundary
router as "ASBR".
There are no instances of multiple equal-cost shortest paths in this
example. Also, since there are no areas, there are no inter-area paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes have
been calculated to routers RT5 and RT7. This allows external routes to
be calculated to the destinations advertised by RT5 and RT7 (i.e.,
networks N12, N13, N14 and N15). It is assumed all AS external
advertisements originated by RT5 and RT7 are advertising type 1 external
metrics. This results in type 1 external paths being calculated to
destinations N12-N15.
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11.3 Sample routing table, with areas
Consider the previous example, this time split into OSPF areas. An OSPF
area configuration is pictured in Figure 6. Router RT4's routing table
will be described for this area configuration. Router RT4 has a
connection to Area 1 and a backbone connection. This causes Router RT4
to view the AS as the concatenation of the two graphs shown in Figures 7
and 8. The resulting routing table is displayed in Table 13.
Again, routers RT5 and RT7 are AS boundary routers. Routers RT3, RT4,
RT7, RT10 and RT11 are area border routers. Note that there are two
routing entries (in this case having identical paths) for router RT7, in
its dual capacities as an area border router and an AS boundary router.
Note also that there are two routing entries for the area border router
RT3, since it has two areas in common with RT4 (Area 1 and the
backbone).
Backbone paths have been calculated to all area border routers (BR).
These are used when determining the inter-area routes. Note that all of
Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)
__________________________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
ASBR RT5 0 intra-area 6 RT5 *
ASBR RT7 0 intra-area 8 RT10 *
__________________________________________________________________________
N N12 * type 1 external 10 RT10 RT7
N N13 * type 1 external 14 RT5 RT5
N N14 * type 1 external 14 RT5 RT5
N N15 * type 1 external 17 RT10 RT7
Table 12: The routing table for Router RT6 (no configured areas).
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the inter-area routes are associated with the backbone; this is always
the case when the router is itself an area border router. Routing
information is condensed at area boundaries. In this example, we assume
that Area 3 has been defined so that networks N9-N11 and the host route
to H1 are all condensed to a single route when advertised to the
backbone (by router RT11). Note that the cost of this route is the
minimum of the set of costs to its individual components.
There is a virtual link configured between routers RT10 and RT11.
Without this configured virtual link, RT11 would be unable to advertise
a route for networks N9-N11 and host H1 into the backbone, and there
would not be an entry for these networks in router RT4's routing table.
In this example there are two equal-cost paths to network N12. However,
they both use the same next hop (Router RT5).
Router RT4's routing table would improve (i.e., some of the paths in the
routing table would become shorter) if an additional virtual link were
configured between router RT4 and router RT3. The new virtual link
would itself be associated with the first entry for area border router
RT3 in Table 13 (an intra-area path through Area 1). This would yield a
cost of 1 for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown in Table
14.
12. Link State Advertisements
Each router in the Autonomous System originates one or more link state
advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
link state advertisements forms the link state or topological database.
Each separate type of advertisement has a separate function. Router
links and network links advertisements describe how an area's routers
and networks are interconnected. Summary link advertisements provide a
way of condensing an area's routing information. AS external
advertisements provide a way of transparently advertising externally-
derived routing information throughout the Autonomous System.
Each link state advertisement begins with a standard 20-byte header.
This link state header is discussed below.
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Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)
_______________________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
BR RT3 1 intra-area 1 * *
_______________________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
BR RT3 0 intra-area 21 RT5 *
BR RT7 0 intra-area 14 RT5 *
BR RT10 0 intra-area 22 RT5 *
BR RT11 0 intra-area 25 RT5 *
ASBR RT5 0 intra-area 8 * *
ASBR RT7 0 intra-area 14 RT5 *
_______________________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 26 RT5 RT11
_______________________________________________________________________________
N N12 * type 1 external 16 RT5 RT5,RT7
N N13 * type 1 external 16 RT5 RT5
N N14 * type 1 external 16 RT5 RT5
N N15 * type 1 external 23 RT5 RT7
Table 13: Router RT4's routing table in the presence of areas.
Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)
__________________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
BR RT3 0 intra-area 1 * *
BR RT10 0 intra-area 16 RT3 *
BR RT11 0 intra-area 19 RT3 *
__________________________________________________________________________
N N9-N11,H1 0 inter-area 20 RT3 RT11
Table 14: Changes resulting from an additional virtual link.
12.1 The Link State Header
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The link state header contains the LS type, Link State ID and
Advertising Router fields. The combination of these three fields
uniquely identifies the link state advertisement.
There may be several instances of an advertisement present in the
Autonomous System, all at the same time. It must then be determined
which instance is more recent. This determination is made be examining
the LS sequence, LS checksum and LS age fields. These fields are also
contained in the 20-byte link state header.
Several of the OSPF packet types list link state advertisements. When
the instance is not important, an advertisement is referred to by its LS
type, Link State ID and Advertising Router (see Link State Request
Packets). Otherwise, the LS sequence number, LS age and LS checksum
fields must also be referenced.
A detailed explanation of the fields contained in the link state header
follows.
12.1.1 LS age
This field is the age of the link state advertisement in seconds. It
should be processed as an unsigned 16-bit integer. It is set to 0 when
the link state advertisement is originated. It must be incremented by
InfTransDelay on every hop of the flooding procedure. Link state
advertisements are also aged as they are held in each router's database.
The age of a link state advertisement is never incremented past MaxAge.
Advertisements having age MaxAge are not used in the routing table
calculation. When an advertisement's age first reaches MaxAge, it is
reflooded. A link state advertisement of age MaxAge is finally flushed
from the database when it is no longer contained on any neighbor Link
state retransmission lists. This indicates that it has been
acknowledged by all adjacent neighbors. For more information on the
aging of link state advertisements, consult Section 14.
Ages are examined when a router receives two instances of a link state
advertisement, both having identical sequence numbers and checksums. An
instance of age MaxAge is then always accepted as most recent; this
allows old advertisements to be flushed quickly from the routing domain.
Otherwise, if the ages differ by more than MaxAgeDiff, the instance
having the smaller age is accepted as most recent.[11] See Section 13.1
for more details.
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12.1.2 Options
The options field in the link state header indicates which optional
capabilities are associated with the advertisement. OSPF's optional
capabilities are described in Section 4.5. There are currently two
optional capabilities defined; they are represented by the T-bit and E-
bit found in the options field. The rest of the options field should be
set to zero.
The E-bit represents OSPF's external routing capability. This bit
should be set in all advertisements associated with the backbone, and
all advertisements associated with non-stub areas (see Section 3.6). It
should also be set in all AS external advertisements. It should be
reset in all router links, network links and summary link advertisements
associated with a stub area. For all link state advertisements, the
setting of the E-bit is for informational purposes only; it does not
affect the routing table calculation.
The T-bit represents OSPF's TOS routing capability. This bit should be
set in a router links advertisement if and only if the router is capable
of calculating separate routes for each IP TOS (see Section 2.4). The
T-bit should always be set in network links advertisements. It should
be set in summary link and AS external link advertisements if and only
if the advertisement describes paths for all TOS values, instead of just
the TOS 0 path. Note that, with the T-bit set, there may still be only
a single metric in the advertisement (the TOS 0 metric). This would
mean that paths for non-zero TOS exist, but are equivalent to the TOS 0
path. A link state advertisement's T-bit is examined when calculating
the routing table's non-zero TOS paths (see Section 16.9).
12.1.3 LS type
The LS type field dictates the format and function of the link state
advertisement. Advertisements of different types have different names
(e.g., router links or network links). All advertisement types, except
the AS external link advertisements (LS type = 5), are flooded
throughout a single area only. AS external link advertisements are
flooded throughout the entire Autonomous System, excluding stub areas
(see Section 3.6). Each separate advertisement type is briefly
described below in Table 15.
LS Type Advertisement description
__________________________________________________
1 These are the router links
advertisements. They describe the
[Moy] [Page 84]RFC 1247 OSPF Version 2 July 1991
LS Type Advertisement description
__________________________________________________
collected states of the router's
interfaces. For more information,
consult Section 12.4.1.
__________________________________________________
2 These are the network links
advertisements. They describe the set
of routers attached to the network. For
more information, consult
Section 12.4.2.
__________________________________________________
3 or 4 These are the summary link
advertisements. They describe
inter-area routes, and enable the
condensation of routing information at
area borders. Originated by area border
routers, the Type 3 advertisements
describe routes to networks while the
Type 4 advertisements describe routes to
AS boundary routers.
__________________________________________________
5 These are the AS external link
advertisements. Originated by AS
boundary routers, they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link
advertisement.
Table 15: OSPF link state advertisements.
12.1.4 Link State ID
This field identifies the piece of the routing domain that is being
described by the advertisement. Depending on the advertisement's LS
type, the Link State ID takes on the values listed in Table 16.
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LS Type Link State ID
______________________________________________________________________
1 The originating router's Router ID.
2 The IP interface address of the network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS boundary router.
5 The destination network's IP address.
Table 16: The advertisement's Link State ID.
When the link state advertisement is describing a network, the Link
State ID is either the network's IP address (as in type 3 summary link
advertisements and in AS external link advertisements) or the network's
IP address is easily derivable from the Link State ID (note that masking
a network links advertisement's Link State ID with the network's subnet
mask yields the network's IP address). When the link state
advertisement is describing a router, the Link State ID is always the
described router's OSPF Router ID.
When an AS external advertisement (LS Type = 5) is describing a default
route, its Link State ID is set to DefaultDestination (0.0.0.0).
12.1.5 Advertising Router
This field specifies the OSPF Router ID of the advertisement's
originator. For router links advertisements, this field is identical to
the Link State ID field. Network link advertisements are originated by
the network's Designated Router. Summary link advertisements are
originated by area border routers. Finally, AS external link
advertisements are originated by AS boundary routers.
12.1.6 LS sequence number
The sequence number field is a signed 32-bit integer. It is used to
detect old and duplicate link state advertisements. The space of
sequence numbers is linearly ordered. The larger the sequence number
(when compared as signed 32-bit integers) the more recent the
advertisement. To describe to sequence number space more precisely, let
N refer in the discussion below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and unused). This
leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)
sequence number. A router uses this sequence number the first time it
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originates any link state advertisement. Afterwards, the
advertisement's sequence number is incremented each time the router
originates a new instance of the advertisement. When an attempt is made
to increment the sequence number past the maximum value of of N - 1
(0x7fffffff), the current instance of the advertisement must first be
flushed from the routing domain. This is done by prematurely aging the
advertisement (see Section 14.1) and reflooding it. As soon as this
flood has been acknowledged by all adjacent neighbors, a new instance
can be originated with sequence number of -N + 1 (0x80000001).
The router may be forced to promote the sequence number of one of its
advertisements when a more recent instance of the advertisement is
unexpectedly received during the flooding process. This should be a
rare event. This may indicate that an out-of-date advertisement,
originated by the router itself before its last restart/reload, still
exists in the Autonomous System. For more information see Section 13.4.
,uh "12.1.7 LS checksum"
This field is the checksum of the complete contents of the
advertisement, excepting the age field. The age field is excepted so
that an advertisement's age can be incremented without updating the
checksum. The checksum used is the same that is used for ISO
connectionless datagrams; it is commonly referred to as the Fletcher
checksum. It is documented in Annex C of [RFC 994]. The link state
header also contains the length of the advertisement in bytes;
subtracting the size of the age field (two bytes) yields the amount of
data to checksum.
The checksum is used to detect data corruption of an advertisement.
This corruption can occur while an advertisement is being flooded, or
while it is being held in a router's memory. The LS checksum field
cannot take on the value of zero; the occurrence of such a value should
be considered a checksum failure. In other words, calculation of the
checksum is not optional.
The checksum of a link state advertisement is verified in two cases: a)
when it is received in a Link State Update Packet and b) at times during
the aging of the link state database. The detection of a checksum
failure leads to separate actions in each case. See Sections 13 and 14
for more details.
Whenever the LS sequence number field indicates that two instances of an
advertisement are the same, the LS checksum field is examined. If there
is a difference, the instance with the larger checksum is considered to
be most recent.[12] See Section 13.1 for more details.
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12.2 The link state database
A router has a separate link state database for every area to which it
belongs. The link state database has been referred to elsewhere in the
text as the topological database. All routers belonging to the same
area have identical topological databases for the area.
The databases for each individual area are always dealt with separately.
The shortest path calculation is performed separately for each area (see
Section 16). Components of the area topological database are flooded
throughout the area only. Finally, when an adjacency (belonging to Area
A) is being brought up, only the database for Area A is synchronized
between the two routers.
The area database is composed of router links advertisements, network
links advertisements, and summary link advertisements (all listed in the
area data structure). In addition, external routes (AS external
advertisements) are included in all non-stub area databases (see Section
3.6).
An implementation of OSPF must be able to access individual pieces of an
area database. This lookup function is based on an advertisement's LS
type, Link State ID and Advertising Router.[13] There will be a single
instance (the most up-to-date) of each link state advertisement in the
database. The database lookup function is invoked during the link state
flooding procedure (Section 13) and the routing table calculation
(Section 16). In addition, using this lookup function the router can
determine whether it has itself ever originated a particular link state
advertisement, and if so, with what LS sequence number.
A link state advertisement is added to a router's database when either
a) it is received during the flooding process (Section 13) or b) it is
originated by the router itself (Section 12.4). A link state
advertisement is deleted from a router's database when either a) it has
been overwritten by a newer instance during the flooding process
(Section 13) or b) the router originates a newer instance of one of its
self-originated advertisements (Section 12.4) or c) the advertisement
ages out and is flushed from the routing domain (Section 14). Whenever
a link state advertisement is deleted from the database it must also be
removed from all neighbors' Link state retransmission lists (see Section
10).
12.3 Representation of TOS
All OSPF link state advertisements (with the exception of network links
advertisements) specify metrics. In router links advertisements, the
metrics indicate the costs of the described interfaces. In summary link
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and AS external link advertisements, the metric indicates the cost of
the described path. In all of these advertisements, a separate metric
can be specified for each IP TOS. TOS is encoded in an OSPF link state
advertisement as the following mapping of the Delay (D), Throughput (T)
and Reliability (R) flags found in the IP packet header's TOS field (see
[RFC 791]).
OSPF encoding D T R
_________________________
0 0 0 0
4 0 0 1
8 0 1 0
12 0 1 1
16 1 0 0
20 1 0 1
24 1 1 0
28 1 1 1
Table 17: Representing TOS in OSPF.
Each OSPF link state advertisement must specify the TOS 0 metric. Other
TOS metrics, if they appear, must appear in order of increasing TOS
encoding. For example, the TOS 8 (high throughput) metric must always
appear before the TOS 16 (low delay) metric when both are specified. If
a metric for some non-zero TOS is not specified, its cost defaults to
the cost for TOS 0, unless the T-bit is reset in the advertisement's
options field (see Section 12.1.2 for more details).
Note that if more TOS types are defined in a future IP architecture,
OSPF's TOS encoding can be extended in a straightforward manner.
12.4 Originating link state advertisements
A router may originate many types of link state advertisements. A
router originates a router links advertisement for each area to which it
belongs. If the router is also the Designated Router for any of its
attached networks, it will originate a network links advertisement for
that network.
Area border routers originate a single summary links advertisement for
each known inter-area destination. AS boundary routers originate a
single AS external links advertisement for each known AS external
destination. Destinations are advertised one at a time so that the
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change in any single route can be flooded without reflooding the entire
collection of routes. During the flooding procedure, many link state
advertisements can be carried by a single Link State Update packet.
As an example, consider router RT4 in Figure 6. It is an area border
router, having a connection to Area 1 and the backbone. Router RT4
originates 5 distinct link state advertisements into the backbone (one
router links, and one summary link for each of the networks N1-N4).
Router RT4 will also originate 8 distinct link state advertisements into
Area 1 (one router links and seven summary link advertisements as
pictured in Figure 7). If RT4 has been selected as Designated Router
for network N3, it will also originate a network links advertisement for
N3 into Area 1.
In this same figure, router RT5 will be originating 3 distinct AS
external link advertisements (one for each of the networks N12-N14).
These will be flooded throughout the entire AS, assuming that none of
the areas have been configured as stubs. However, if area 3 has been
configured as a stub area, the external advertisements for networks
N12-N14 will not be flooded into area 3 (see Section 3.6). Instead,
router RT11 would originate a default summary link advertisement that
would be flooded throughout area 3 (see Section 12.4.3). This instructs
all of area 3's internal routers to send their AS external traffic to
RT11.
Whenever a new instance of a link state advertisement is originated, its
LS sequence number is incremented, its LS age is set to 0, its LS
checksum is calculated, and the advertisement is added to the link state
database and flooded out the appropriate interfaces. See Section 13.2
for details concerning the installation of the advertisement into the
link state database. See Section 13.3 for details concerning the
flooding of newly originated advertisements.
The eight events that cause a new instance of a link state advertisement
to be originated are:
(1) The LS refresh timer firing. There is a LS refresh timer for each
link state advertisement that the router has originated. The LS
refresh timer is an interval timer, with length LSRefreshTimer. The
LS refresh timer guarantees periodic originations regardless of any
other events that cause new instances. This periodic updating of
link state advertisements adds robustness to the link state
algorithm. Link state advertisements that solely describe
unreachable destinations should not be refreshed, but should instead
be flushed from the routing domain (see Section 14.1).
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When whatever is being described by a link state advertisement changes,
a new advertisement is originated. Two instances of the same link state
advertisement may not be originated within the time period
MinLSInterval. This may require that the generation of the next
instance to be delayed by up to MinLSInterval. The following changes
may cause a router to originate a new instance of an advertisement.
These changes should cause new originations only if the contents of the
new advertisement would be different.
(2) An interface's state changes (see Section 9.1). This may mean that
it is necessary to produce a new instance of the router links
advertisement.
(3) An attached network's Designated Router changes. A new router links
advertisement should be originated. Also, if the router itself is
now the Designated Router, a new network links advertisement should
be produced.
(4) One of the neighboring routers changes to/from the FULL state. This
may mean that it is necessary to produce a new instance of the
router links advertisement. Also, if the router is itself the
Designated Router for the attached network, a new network links
advertisement should be produced.
The next three events concern area border routers only.
(5) An intra-area route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
area (this includes the backbone).
(6) An inter-area route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
area (but NEVER for the backbone).
(7) The router becomes newly attached to an area. The router must then
originate summary link advertisements into the newly attached area
for all pertinent intra-area and inter-area routes in its routing
table. See Section 12.4.3 for more details.
The last event concerns AS boundary routers only.
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(8) An external route gained through direct experience with an external
routing protocol (like EGP) changes. This will cause the AS
boundary router to originate a new instance of an external links
advertisement.
The construction of each type of the link state advertisement is
explained below. In general, these sections describe the contents of
the advertisement body (i.e., the part coming after the 20-byte
advertisement header). For information concerning the building of the
link state advertisement header, see Section 12.1.
12.4.1 Router links
A router originates a router links advertisement for each area that it
belongs to. Such an advertisement describes the collected states of the
router's links to the area. The advertisement is flooded throughout the
particular area, and no further.
The format of a router links advertisement is shown in Appendix A
(Section A.4.2). The first 20 bytes of the advertisement consist of the
generic link state header that was discussed in Section 12.1. Router
links advertisements have LS type = 1. The router indicates whether it
is willing to calculate separate routes for each IP TOS by setting (or
resetting) the T-bit of the link state advertisement's Options field.
A router also indicates whether it is an area border router, or an AS
boundary router, by setting the appropriate bits in its router links
advertisements. This enables paths to those types of routers to be
saved in the routing table, for later processing of summary link
advertisements and AS external link advertisements.
The router links advertisement then describes the router's working
connections (links) to the area. Each link is typed according to the
_________________________________________
(Figure not included in text version.)
Figure 15: Area 1 with IP addresses shown
Figure 16: Forwarding address example
_________________________________________
[Moy] [Page 92]RFC 1247 OSPF Version 2 July 1991
kind of attached network. Each link is also labelled with its Link ID.
This ID gives a name to the entity that is on the other end of the link.
Table 18 summarizes the values used for the type and Link ID fields.
Link type Description Link ID
____________________________________________________________________________
1 Point-to-point link Neighbor Router ID
2 Link to transit network Interface address of Designated Router
3 Link to stub network IP network number
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the router links advertisement.
In addition, the Link Data field is specified for each link. This field
gives 32 bits of extra information for the link. For links to routers
and transit networks, this field specifies the IP interface address of
the associated router interface (this is needed by the routing table
calculation, see Section 16.3). For links to stub networks, this field
specifies the network's IP address mask.
Finally, the cost of using the link for output (possibly specifying a
different cost for each type of service) is specified. The output cost
of a link is configurable. It must always be non-zero.
To further describe the process of building the list of link records,
suppose a router wishes to build router links advertisement for an Area
A. The router examines its collection of interface data structures.
For each interface, the following steps are taken:
o If the attached network does not belong to Area A, no links are
added to the advertisement, and the next interface should be
examined.
o Else, if the state of the interface is Down, no links are added.
o Else, if the state of the interface is Point-to-Point, then add
links according to the following:
- If the neighboring router is fully adjacent, add a Type 1 link
(point-to-point) if this is an interface to a point-to-point
network, or add a type 4 link (virtual link) if this is a
virtual link. The Link ID should be set to the Router ID of the
neighboring router, and the Link Data should specify the
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interface IP address.
- If this is a numbered point-to-point network (i.e, not a virtual
link and not an unnumbered point-to-point network) and the
neighboring router's IP address is known, add a Type 3 link
(stub network) whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host route, and
whose cost is the interface's configured output cost.
o Else if the state of the interface is Loopback, add a Type 3 link
(stub network) as long as this is not an interface to an unnumbered
serial line. The Link ID should be set to the IP interface address,
the Link Data set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.
o Else if the state of the interface is Waiting, add a Type 3 link
(stub network) whose Link ID is the IP network number of the
attached network and whose Link Data is the attached network's
address mask.
o Else, there has been a Designated Router selected for the attached
network. If the router is fully adjacent to the Designated Router,
or if the router itself is Designated Router and is fully adjacent
to at least one other router, add a single Type 2 link (transit
network) whose whose link ID is the IP interface address of the
attached network's Designated Router (which may be the router
itself) and whose Link Data is the interface IP address. Otherwise,
add a link as if the interface state were Waiting (see above).
Unless otherwise specified, the cost of each link generated by the above
procedure is equal to the output cost of the associated interface. Note
that in the case of serial lines, multiple links may be generated by a
single interface.
After consideration of all the router interfaces, host links are added
to the advertisement by examining the list of attached hosts. A host
route is represented as a Type 3 link (stub network) whose link ID is
the host's IP address and whose Link Data is the mask of all ones
(0xffffffff).
As an example, consider the router links advertisements generated by
router RT3, as pictured in Figure 6. The area containing router RT3
(Area 1) has been redrawn, with actual network addresses, in Figure 15.
Assume that the last byte of all of RT3's interface addresses is 3,
giving it the interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes. In addition, assume that
all links are functional, and that Router IDs are assigned as the
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smallest IP interface address.
RT3 originates two router links advertisements, one for Area 1 and one
for the backbone. Assume that router RT4 has been selected as the
Designated router for network 192.1.1.0. RT3's router links
advertisement for Area 1 is then shown below. It indicates that RT3 has
two connections to Area 1, the first a link to the transit network
192.1.1.0 and the second a link to the stub network 192.1.4.0. Note
that the transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is the Designated
Router RT4's IP interface to 192.1.1.0). Note also that RT3 has
indicated that it is capable of calculating separate routes based on IP
TOS, through setting the T-bit in the Options field. It has also
indicated that it is an area border router.
; RT3's router links advertisement for Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Designated Router
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# other metrics = 0
TOS 0 metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# other metrics = 0
TOS 0 metric = 2
Next RT3's router links advertisement for the backbone is shown. It
indicates that RT3 has a single attachment to the backbone. This
attachment is via an unnumbered point-to-point link to router RT6. RT3
has again indicated that it is TOS-capable, and that it is an area
border router.
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
[Moy] [Page 95]RFC 1247 OSPF Version 2 July 1991
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.0 ;Interface to unnumbered SL
Type = 1 ;connects to router
# other metrics = 0
TOS 0 metric = 8
Even though router RT3 has indicated that it is TOS-capable in the above
examples, only a single metric (the TOS 0 metric) has been specified for
each interface. Different metrics can be specified for each TOS. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3.
As an example, suppose the point-to-point link between routers RT3 and
RT6 in Figure 15 is a satellite link. The AS administrator may want to
encourage the use of the line for high bandwidth traffic. This would be
done by setting the metric artificially low for that TOS. Router RT3
would then originate the following router links advertisement for the
backbone (IP TOS 8 = high bandwidth):
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 1
Link ID = 18.10.0.6 ; Neighbor's Router ID
Link Data = 0.0.0.0 ;Interface to unnumbered SL
Type = 1 ;connects to router
# other metrics = 1
TOS 0 metric = 8
TOS = 8 ;High bandwidth
metric = 1 ;traffic preferred
12.4.2 Network links
A network links advertisement is generated for every transit multi-
access network. (A transit network is a network having two or more
attached routers). The network links advertisement describes all the
[Moy] [Page 96]RFC 1247 OSPF Version 2 July 1991
routers that are attached to the network.
The Designated Router for the network originates the advertisement. The
Designated Router originates the advertisement only if it is fully
adjacent to at least one other router on the network. The network links
advertisement is flooded throughout the area that contains the transit
network, and no further. The networks links advertisement lists those
routers that are fully adjacent to the Designated Router; each fully
adjacent router is identified by its OSPF Router ID. The Designated
Router includes itself in this list.
The Link State ID for a network links advertisement is the IP interface
address of the Designated Router. This value, masked by the network's
address mask (which is also contained in the network links
advertisement) yields the network's IP address.
A router that has formerly been the Designated Router for a network, but
is no longer, should flush the network links advertisement that it had
previously originated. This advertisement is no longer used in the
routing table calculation. It is flushed by prematurely incrementing
the advertisement's age to MaxAge and reflooding (see Section 14.1).
As an example of a network links advertisement, again consider the area
configuration in Figure 6. Network links advertisements are originated
for network N3 in Area 1, networks N6 and N8 in Area 2, and network N9
in Area 3. Assuming that router RT4 has been selected as the Designated
Router for network N3, the following network links advertisement is
generated by RT4 on behalf of network N3 (see Figure 15 for the address
assignments):
; network links advertisement for network N3
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 2 ;indicates network links
Link State ID = 192.1.1.4 ;IP address of Designated Router
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID
12.4.3 Summary links
Each summary link advertisement describes a route to a single
destination. Summary link advertisements are flooded throughout a
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single area only. The destination described is one that is external to
the area, yet still belonging to the Autonomous System.
The DefaultDestination can also be specified in summary link
advertisements. This is used when implementing OSPF's stub area
functionality (see Section 3.6). In a stub area, instead of importing
external routes each area border router originates a "default summary
link" (Link State ID = DefaultDestination) into the area.
Summary link advertisements are originated by area border routers. The
precise summary routes to advertise into an area are determined by
examining the routing table structure (see Section 11). Only intra-area
routes are advertised into the backbone. Both intra-area and inter-area
routes are advertised into the other areas.
To determine which routes to advertise into an attached Area A, each
routing table entry is processed as follows:
o Only Destination types of network and AS boundary router are
advertised in summary link advertisements. If the routing table
entry's Destination type is area border router, examine the next
routing table entry.
o AS external routes are never advertised in summary link
advertisements. If the routing table entry has Path-type type 1
external or type 2 external, examine the next routing table entry.
o Else, if the area associated with this set of paths is the Area A
itself, do not generate a summary link advertisement for the
route.[14]
o Else, if the destination of this route is an AS boundary router,
generate a Type 4 link state advertisement for the destination, with
Link State ID equal to the AS boundary router's ID and metric equal
to the routing table entry's cost. These advertisements should not
be generated if area A has been configured as a stub area.
o Else, the Destination type is network. If this is an inter-area
route, generate a Type 3 advertisement for the destination, with
Link State ID equal to the network's address and metric equal to the
routing table cost.
o The one remaining case is an intra-area route to a network. This
means that the network is contained in one of the router's directly
attached areas. In general, this information must be condensed
before appearing in summary link advertisements. Remember that an
area has been defined as a list of address ranges, each range
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consisting of an [address,mask] pair. A single Type 3 advertisement
must be made for each range, with Link State ID equal to the range's
address and cost equal to the smallest cost of any of the component
networks.
If virtual links are being used to provide/increase connectivity of
the backbone, routing information concerning the backbone networks
should not be condensed before being summarized into the virtual
links' transit areas. In other words, the backbone ranges should be
ignored when originating summary links into these areas. The
existence of virtual links can be determined during the shortest
path calculation for the backbone (see Section 16.1).
In addition, if area A has been configured as a stub area and the router
is an area border router, it should advertise a default summary link
into Area A. The Link State ID for the advertisement should be set to
DefaultDestination, and the metric set to the (per-area) configurable
parameter StubDefaultCost.
If a router advertises a summary advertisement for a destination which
then becomes unreachable, the router must then flush the advertisement
from the routing domain by setting its age to MaxAge and reflooding (see
Section 14.1). Also, if the destination is still reachable, yet can no
longer be advertised according to the above procedure (e.g., it is now
an inter-area route, when it used to be an intra-area route associated
with some non-backbone area; it would thus no longer be advertisable to
the backbone), the advertisement should also be flushed from the routing
domain.
For an example of summary link advertisements, consider again the area
configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all
area border routers, and therefore are originating summary links
advertisements. Consider in particular router RT4. Its routing table
was calculated as the example in Section 11.3. RT4 originates summary
link advertisements into both the backbone and Area 1. Into the
backbone, router RT4 originates separate advertisements for each of the
networks N1-N4. Into Area 1, router RT4 originates separate
advertisements for networks N6-N8 and the AS boundary routers RT5,RT7.
It also condenses host routes Ia and Ib into a single summary
advertisement. Finally, the routes to networks N9,N10,N11 and host H9
are advertised by a single summary link. This condensation was
originally performed by the router RT11.
These advertisements are illustrated graphically in Figures 7 and 8.
Two of the summary link advertisements originated by router RT4 follow.
The actual IP addresses for the networks and routers in question have
been assigned in Figure 15.
[Moy] [Page 99]RFC 1247 OSPF Version 2 July 1991
; summary link advertisement for network N1,
; originated by router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 3 ;indicates summary link to IP net
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 4
; summary link advertisement for AS boundary router RT7
; originated by router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 4 ;indicates summary link to ASBR
Link State ID = router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 14
Summary link advertisements pertain to a single destination (IP network
or AS boundary router). However, for a single destination there may be
separate sets of paths, and therefore separate routing table entries,
for each Type of Service. All these entries must be considered when
building the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they exist) for each
TOS. The encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
Clearing the T-bit in the Options field of a summary link advertisement
indicates that there is a TOS 0 path to the destination, but no paths
for non-zero TOS. This can happen when non-TOS capable routers exist in
the routing domain (see Section 2.4).
12.4.4 AS external links
AS external link advertisements describe routes to destinations external
to the Autonomous System. Most AS external link advertisements describe
routes to specific external destinations. However, a default route for
the Autonomous System can be described in an AS external advertisement
by setting the advertisement's Link State ID to DefaultDestination
(0.0.0.0). AS external link advertisements are originated by AS
boundary routers. An AS boundary router originates a single AS external
link advertisement for each external route that it has learned, either
through another routing protocol (such as EGP), or through configuration
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information.
In general, AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
single area. However, AS external advertisements are not flooded
into/throughout stub areas (see Section 3.6). This enables a reduction
in link state database size for routers internal to stub areas.
The metric that is advertised for an external route can be one of two
types. Type 1 metrics are comparable to the link state metric. Type 2
metrics are assumed to be larger than the cost of any intra-AS path. As
with summary link advertisements, if separate paths exist based on TOS,
separate TOS costs can be included in the AS external link
advertisement. The encoding of TOS in OSPF link state advertisements is
described in Section 12.3. If the T-bit of the advertisement's Options
field is clear, no non-zero TOS paths to the destination exist.
If a router advertises an AS external link advertisement for a
destination which then becomes unreachable, the router must then flush
the advertisement from the routing domain by setting its age to MaxAge
and reflooding (see Section 14.1).
For an example of AS external link advertisements, consider once again
the AS pictured in Figure 6. There are two AS boundary routers: RT5 and
RT7. Router RT5 originates three external link advertisements, for
networks N12-N14. Router RT7 originates two external link
advertisements, for networks N12 and N15. Assume that RT7 has learned
its route to N12 via EGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the following advertisement
for N12:
; AS external link advertisement for network N12,
; originated by router RT7
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 2
Forwarding address = 0.0.0.0
In the above example, the forwarding address field has been set to
0.0.0.0, indicating that packets for the external destination should be
forwarded to the advertising OSPF router (RT7). This is not always
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desirable. Consider the example pictured in Figure 16. There are three
OSPF routers (RTA, RTB and RTC) connected to a common network. Only one
of these routers, RTA, is exchanging EGP information with the non-OSPF
router RTX. RTA must then originate AS external link state
advertisements for those destinations it has learned from RTX. By using
the AS external advertisement's forwarding address field, RTA can
specify that packets for these destinations be forwarded directly to
RTX. Without this feature, routers RTB and RTC would take an extra hop
to get to these destinations.
Note that when the forwarding address field is non-zero, it should point
to a router belonging to another Autonomous System.
A forwarding address can also be specified for the default route. For
example, in figure 16 RTA may want to specify that all externally-
destined packets should by default be forwarded to its EGP peer RTX.
The resulting AS external link advertisement is pictured below. Note
that the Link State ID is set to DefaultDestination.
; Default route, originated by router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange EGP
information with RTX. In this case, RTA and RTB would originate the
same set of external advertisements. These advertisements, if they
specify the same metric, would be functionally equivalent since they
would specify the same destination and forwarding address (RTX). This
leads to a clear duplication of effort. If only one of RTA or RTB
originated the set of external advertisements, the routing would remain
the same, and the size of the link state database would decrease.
However, it must be unambiguously defined as to which router originates
the advertisements (otherwise neither may, or the identity of the
originator may oscillate). The following rule is thereby established:
if two routers, both reachable from one another, originate functionally
equivalent AS external advertisements (i.e., same destination, cost and
non-zero forwarding address), then the advertisement originated by the
router having the highest OSPF Router ID is used. The router having the
lower OSPF Router ID can then flush its advertisement. Flushing a link
[Moy] [Page 102]RFC 1247 OSPF Version 2 July 1991
state advertisement is discussed in Section 14.1.
13. The Flooding Procedure
Link State Update packets provide the mechanism for flooding link state
advertisements. A Link State Update packet may contain several distinct
advertisements, and floods each advertisement one hop further from its
point of origination. To make the flooding procedure reliable, each
advertisement must be acknowledged separately. Acknowledgments are
transmitted in Link State Acknowledgment packets. Many separate
acknowledgments can be grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has been
received. Many consistency checks have been made on the received packet
before being handed to the flooding procedure (see Section 8.2). In
particular, the Link State Update packet has been associated with a
particular neighbor, and a particular area. If the neighbor is in a
lesser state than Exchange, the packet should be dropped without further
processing.
All types of link state advertisements, other than AS external links,
are associated with a specific area. However, link state advertisements
do not contain an area field. A link state advertisement's area must be
deduced from the Link State Update packet header.
For each link state advertisement contained in the packet, the following
steps are taken:
(1) Validate the advertisement's link state checksum. If the checksum
turns out to be invalid, discard the advertisement and get the next
one from the Link State Update packet.
(2) Examine the link state advertisement's LS type. If the LS type is
unknown, discard the advertisement and get the next one from the
Link State Update Packet. This specification defines LS Types 1-5
(see Section 4.3).
(3) Else if this is a AS external advertisement (LS type = 5), and the
area has been configured as a stub area, discard the advertisement
and get the next one from the Link State Update Packet. AS external
advertisements are not flooded into/throughout stub areas (see
Section 3.6).
(4) Else if the advertisement's age is equal to MaxAge, and there is
currently no instance of the advertisement in the router's link
state database, then take the following actions:
[Moy] [Page 103]RFC 1247 OSPF Version 2 July 1991
(a) Acknowledge the receipt of the advertisement by sending a Link
State Acknowledgment packet back to the sending neighbor (see
Section 13.5).
(b) Purge all outstanding requests for equal or previous instances
of the advertisement from the sending neighbor's Link State
Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge advertisement in the link state
database. Otherwise, simply discard the advertisement. In
either case, examine the next advertisement (if any) listed in
the Link State Update packet.
(5) Otherwise, find the instance of this advertisement that is currently
contained in the router's link state database. If there is no
database copy, or the received advertisement is more recent than the
database copy (see Section 13.1 below for the determination of which
advertisement is more recent) the following steps must be performed:
(a) If there is already a database copy, and if the database copy
was installed less than MinLSInterval seconds ago, discard the
new advertisement (without acknowledging it) and examine the
next advertisement (if any) listed in the Link State Update
packet.
(b) Otherwise immediately flood the new advertisement out some
subset of the router's interfaces (see Section 13.3). In some
cases (e.g., the state of the receiving interface is DR and the
advertisement was received from a router other than the Backup
DR) the advertisement will be flooded back out the receiving
interface. This occurrence should be noted for later use by the
acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link state
retransmission lists.
(d) Install the new advertisement in the link state database
(replacing the current database copy). This may cause the
routing table calculation to be scheduled. In addition,
timestamp the new advertisement with the current time (i.e., the
time it was received). The flooding procedure cannot overwrite
the newly installed advertisement until MinLSInterval seconds
have elapsed. The advertisement installation process is
discussed further in Section 13.2.
(e) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
[Moy] [Page 104]RFC 1247 OSPF Version 2 July 1991
interface. This is explained below in Section 13.5.
(f) If this new link state advertisement indicates that it was
originated by this router itself, the router must advance the
advertisement's link state sequence number, and issue a new
instance of the advertisement (see Section 13.4).
(6) Else, if there is an instance of the advertisement on the sending
neighbor's Link state request list, an error has occurred in the
Database Description process. In this case, restart the Database
Description process by generating the neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.
(7) Else, if the received advertisement is the same instance as the
database copy (i.e., neither one is more recent) the following two
steps should be performed:
(a) If the advertisement is listed in the Link state retransmission
list for the receiving adjacency, the router itself is expecting
an acknowledgment for this advertisement. The router should
treat the received advertisement as an acknowledgment, by
removing the advertisement from the Link state retransmission
list. This is termed an "implied acknowledgment". Its
occurrence should be noted for later use by the acknowledgment
process (Section 13.5).
(b) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(8) Else, the database copy is more recent. Note an unusual event to
network management, discard the advertisement and process the next
link state advertisement contained in the packet.
13.1 Determining which link state is newer
When a router encounters two instances of a link state advertisement, it
must determine which is more recent. This occurred above when comparing
a received advertisement to the database copy. This comparison must
also be done during the database exchange procedure which occurs during
adjacency bring-up.
A link state advertisement is identified by its LS type, Link State ID
and Advertising Router. For two instances of the same advertisement,
the LS sequence number, LS age, and LS checksum fields are used to
determine which instance is more recent:
[Moy] [Page 105]RFC 1247 OSPF Version 2 July 1991
o The advertisement having the newer LS sequence number is more
recent. See Section 12.1.6 for an explanation of the LS sequence
number space. If both instances have the same LS sequence number,
then:
o If the two instances have different LS checksums, then the instance
having the larger LS checksum (when considered as a 16-bit unsigned
integer) is considered more recent.
o Else, if only one of the instances is of age MaxAge, the instance of
age MaxAge is considered to be more recent.
o Else, if the ages of the two instances differ by more than
MaxAgeDiff, the instance having the smaller (younger) age is
considered to be more recent.
o Else, the two instances are considered to be identical.
13.2 Installing link state advertisements in the database
Installing a new link state advertisement in the database, either as the
result of flooding or a newly self originated advertisement, may cause
the routing table structure to be recalculated. The contents of the new
advertisement should be compared to the old instance, if present. If
there is no difference, there is no need to recalculate the routing
table. (Note that even if the contents are the same, the LS checksum
will probably be different, since the checksum covers the LS sequence
number.)
If the contents are different, the following pieces of the routing table
must be recalculated, depending on the LS type field:
Router links, network links
The entire routing table must be recalculated, starting with the
shortest path calculations for each area (not just the area whose
topological database has changed). The reason that the shortest
path calculation cannot be restricted to the single changed area has
to do with the fact that AS boundary routers may belong to multiple
areas. A change in the area currently providing the best route may
force the router to use an intra-area route provided by a different
area.[15]
Summary link
The best route to the destination described by the summary link
advertisement must be re-examined (see Section 16.5). If this
destination is an AS boundary router, it may also be necessary to
[Moy] [Page 106]RFC 1247 OSPF Version 2 July 1991
re-examine all the AS external link advertisements.
AS external link
The best route to the destination described by the AS external link
advertisement must be re-examined (see Section 16.6).
Also, any old instance of the advertisement must be removed from the
database when the new advertisement is installed. This old instance
must also be removed from all neighbors' Link state retransmission lists
(see Section 10).
13.3 Next step in the flooding procedure
When a new (and more recent) advertisement has been received, it must be
flooded out some set of the router's interfaces. This section describes
the second part of flooding procedure (the first part being the
processing that occurred in Section 13), namely, selecting the outgoing
interfaces and adding the advertisement to the appropriate neighbors'
Link state retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link state
request lists.
This section is equally applicable to the flooding of an advertisement
that the router itself has just originated (see Section 12.4). For
these advertisements, this section provides the entirety of the flooding
procedure (i.e., the processing of Section 13 is not performed, since,
for example, the advertisement has not been received from a neighbor and
therefore does not need to be acknowledged).
Depending upon the advertisement's LS type, the advertisement can be
flooded out only certain interfaces. These interfaces, defined by the
following, are called the eligible interfaces:
AS external links (LS Type = 5)
AS external links are flooded throughout the entire AS, with the
exception of stub areas (see Section 3.6). The eligible interfaces
are all the router's interfaces, excluding virtual links and those
interfaces attaching to stub areas.
All other types
All other types are specific to a single area (Area A). The
eligible interfaces are all those interfaces attaching to the Area
A. If Area A is the backbone, this includes all the virtual links.
[Moy] [Page 107]RFC 1247 OSPF Version 2 July 1991
Link state databases must remain synchronized over all adjacencies
associated with the above eligible interfaces. This is accomplished by
executing the following steps on each eligible interface. It should be
noted that this procedure may decide not to flood a link state
advertisement out a particular interface, if there is a high probability
that the attached neighbors have already received the advertisement.
However, in these cases the flooding procedure must be absolutely sure
that the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list. For each eligible interface:
(1) Each of the neighbors attached to this interface are examined, to
determine whether they must receive the new advertisement. The
following steps are executed for each neighbor:
(a) If the neighbor is in a lesser state than Exchange, it does not
participate in flooding, and the next neighbor should be
examined.
(b) Else, if the adjacency is not yet full (neighbor state is
Exchange or Loading), examine the Link state request list
associated with this adjacency. If there is an instance of the
new advertisement on the list, it indicates that the neighboring
router has an instance of the advertisement already. Compare
the new advertisement to the neighbor's copy:
o If the new advertisement is less recent, then try the next
neighbor.
o If the two copies are the same instance, then delete the
advertisement from the Link state request list, and try the
next neighbor.[16]
o Else, the new advertisement is more recent. Delete the
advertisement from the Link state request list.
(c) If the new advertisement was received from this neighbor, try
the next neighbor.
(d) At this point we are not positive that the new neighbor has an
up-to-date instance of this new advertisement. Add the new
advertisement to the Link state retransmission list for the
adjacency. This ensures that the flooding procedure is
reliable; the advertisement will be retransmitted at intervals
until an acknowledgment is seen from the neighbor.
[Moy] [Page 108]RFC 1247 OSPF Version 2 July 1991
(2) The router must now decide whether to flood the new link state
advertisement out this interface. If in the previous step, the link
state advertisement was NOT added to any of the Link state
retransmission lists, there is no need to flood the advertisement
and the next interface should be examined.
(3) If the new advertisement was received on this interface, and it was
received from either the Designated Router or the Backup Designated
Router, chances are all the neighbors have received the
advertisement already. Therefore, examine the next interface.
(4) If the new advertisement was received on this interface, and the
interface state is Backup (i.e., the router itself is the Backup
Designated Router), examine the next interface. The Designated
Router will do the flooding on this interface. If the Designated
Router fails, this router will end up retransmitting the updates.
(5) If this step is reached, the advertisement must be flooded out the
interface. Send a Link State Update packet (with the new
advertisement as contents) out the interface. The advertisement's
LS age must be incremented by InfTransDelay (which must be > 0) when
copied into the outgoing packet (until the LS age field reaches its
maximum value of MaxAge).
On broadcast networks, the Link State Update packets are multicast.
The destination IP address specified for the Link State Update
Packet depends on the state of the interface. If the interface
state is DR or Backup, the address AllSPFRouters should be used.
Otherwise, the address AllDRouters should be used.
On non-broadcast, multi-access networks, separate Link State Update
packets must be sent, as unicasts, to each adjacent neighbor (i.e.,
those in state Exchange or greater). The destination IP addresses
for these packets are the neighbors' IP addresses.
13.4 Receiving self-originated link state
It is a common occurrence to receive a self-originated link state
advertisement via the flooding procedure. If the advertisement received
is a newer instance than the last instance that the router actually
originated, the router must take special action.
The reception of such an advertisement indicates that there are link
state advertisements in the routing domain that were originated before
the last time the router was restarted. In this case, the router must
advance the sequence number for the advertisement one past the received
sequence number, and originate a new instance of the advertisement.
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Note also that if the type of the advertisement is Summary link or AS
external link, the router may no longer have an (advertisable) route to
the destination. In this case, the advertisement should be flushed from
the routing domain by incrementing the advertisement's LS age to MaxAge
and reflooding (see Section 14.1).
13.5 Sending Link State Acknowledgment packets
Each newly received link state advertisement must be acknowledged. This
is usually done by sending Link State Acknowledgment packets. However,
acknowledgments can also be accomplished implicitly by sending Link
State Update packets (see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link State
Acknowledgment packet. Such a packet is sent back out the interface
that has received the advertisements. The packet can be sent in one of
two ways: delayed and sent on an interval timer, or sent directly (as a
unicast) to a particular neighbor. The particular acknowledgment
strategy used depends on the circumstances surrounding the receipt of
the advertisement.
Sending delayed acknowledgments accomplishes several things: it
facilitates the packaging of multiple acknowledgments in a single
packet; it enables a single packet to indicate acknowledgments to
several neighbors at once (through multicasting); and it randomizes the
acknowledgment packets sent by the various routers attached to a multi-
access network. The fixed interval between a router's delayed
transmissions must be short (less than RxmtInterval) or needless
retransmissions will ensue.
Direct acknowledgments are sent to a particular neighbor in response to
the receipt of duplicate link state advertisements. These
acknowledgments are sent as unicasts, and are sent immediately when the
duplicate is received.
The precise procedure for sending Link State Acknowledgment packets is
described in Table 19. The circumstances surrounding the receipt of the
advertisement are listed in the left column. The acknowledgment action
then taken is listed in one of the two right columns. This action
depends on the state of the concerned interface; interfaces in state
Backup behave differently from interfaces in all other states.
Action taken in state
Circumstances Backup All other states
______________________________________________________________
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Action taken in state
Circumstances Backup All other states
______________________________________________________________
Advertisement has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
______________________________________________________________
Advertisement is Delayed ack- Delayed ack-
more recent than nowledgment sent nowledgment sent.
database copy, but if advertisement
was not flooded received from DR,
back out receiving otherwise do noth-
interface ing
______________________________________________________________
Advertisement is a Delayed ack- No acknowledgment
duplicate, and was nowledgment sent sent.
treated as an im- if advertisement
plied acknowledg- received from DR,
ment (see Section otherwise do noth-
13, step 7a). ing
______________________________________________________________
Advertisement is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
______________________________________________________________
Advertisement's age Direct acknowledg- Direct acknowledg-
is equal to MaxAge, ment sent. ment sent.
and there is no
current instance of
the advertisement in
the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgements.
Delayed acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment packets as
multicasts. The Destination IP address used depends on the state of the
interface. If the state is DR or Backup, the destination AllSPFRouters
is used. In other states, the destination AllDRouters is used. On
non-broadcast networks, delayed acks must be unicast separately over
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each adjacency (neighbor whose state is >= Exchange).
The reasoning behind sending the above packets as multicasts is best
explained by an example. Consider the network configuration depicted in
Figure 15. Suppose RT4 has been elected as DR, and RT3 as Backup for
the network N3. When router RT4 floods a new advertisement to network
N3, it is received by routers RT1, RT2, and RT3. These routers will not
flood the advertisement back onto net N3, but they still must ensure
that their topological databases remain synchronized with their adjacent
neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment
from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments
from RT1 and RT2. This is best achieved by sending the acknowledgments
as multicasts.
The reason that the acknowledgment logic for Backup DRs is slightly
different is because they perform differently during the flooding of
link state advertisements (see Section 13.3, step 4).
13.6 Retransmitting link state advertisements
Advertisements flooded out an adjacency are placed on the adjacency's
Link state retransmission list. In order to ensure that flooding is
reliable, these advertisements are retransmitted until they are
acknowledged. The length of time between retransmissions is a
configurable per-interface value, RxmtInterval. If this is set too low
for an interface, needless retransmissions will ensue. If the value is
set too high, the speed of the flooding, in the face of lost packets,
may be affected.
Several retransmitted advertisements may fit into a single Link State
Update packet. When advertisements are to be retransmitted, only the
number fitting in a single Link State Update packet should be
transmitted. Another packet of retransmissions can be sent when some of
the advertisements are acknowledged, or on the next firing of the
retransmission timer.
Link State Update Packets carrying retransmissions are always sent as
unicasts (directly to the physical address of the neighbor). They are
never sent as multicasts. Each advertisement's LS age must be
incremented by InfTransDelay (which must be > 0) when copied into the
outgoing packet (until the LS age field reaches its maximum value of
MaxAge).
If the adjacent router goes down, retransmissions may occur until the
adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is
destroyed, the Link state retransmission list is cleared.
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13.7 Receiving link state acknowledgments
Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding procedure. In
particular, it has been associated with a particular neighbor. If this
neighbor is in a lesser state than Exchange, the packet is discarded.
Otherwise, for each acknowledgment in the packet, the following steps
are performed:
o Does the advertisement acknowledged have an instance on the Link
state retransmission list for the neighbor? If not, examine the
next acknowledgment. Otherwise:
o If the acknowledgment is for the same instance that is contained on
the list, remove the item from the list and examine the next
acknowledgment. Otherwise:
o Log the questionable acknowledgment, and examine the next one.
14. Aging The Link State Database
Each link state advertisement has an age field. The age is expressed in
seconds. An advertisement's age field is incremented while it is
contained in a router's database. Also, when copied into a Link State
Update Packet for flooding out a particular interface, the
advertisement's age is incremented by InfTransDelay.
An advertisement's age is never incremented past the value MaxAge.
Advertisements having age MaxAge are not used in the routing table
calculation. As a router ages its link state database, an
advertisement's age may reach MaxAge.[17] At this time, the router must
attempt to flush the advertisement from the routing domain. This is
done simply by reflooding the MaxAge advertisement just as if it was a
newly originated advertisement (see Section 13.3).
When a Database summary list for a newly adjacent neighbor is formed,
any MaxAge advertisements present in the link state database are added
to the neighbor's Link state retransmission list instead of the
neighbor's Database summary list. See Section 10.3 for more details.
A MaxAge advertisement is removed entirely from the router's link state
database when a) it is no longer contained on any neighbor Link state
retransmission lists and b) none of the router's neighbors are in states
Exchange or Loading.
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When, in the process of aging the link state database, an
advertisement's age hits a multiple of CheckAge, its checksum should be
verified. If the checksum is incorrect, a program or memory error has
been detected, and at the very least the router itself should be
restarted.
14.1 Premature aging of advertisements
A link state advertisement can be flushed from the routing domain by
setting its age to MaxAge and reflooding the advertisement. This
procedure follows the same course as flushing an advertisement whose age
has naturally reached the value MaxAge (see Section 14). In particular,
the MaxAge advertisement is removed from the router's link state
database as soon as a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors are in
states Exchange or Loading. We call the setting of an advertisement's
age to MaxAge premature aging.
Premature aging is used when it is time for a self-originated
advertisement's sequence number field to wrap. At this point, the
current advertisement instance (having LS sequence number of 0x7fffffff)
must be prematurely aged and flushed from the routing domain before a
new instance with sequence number 0x80000001 can be originated. See
Section 12.1.6 for more information.
Premature aging can also be used when, for example, one of the router's
previously advertised external routes is no longer reachable. In this
circumstance, the router can flush its external advertisement from the
routing domain via premature aging. This procedure is preferable to the
alternative, which is to originate a new advertisement for the
destination specifying a metric of LSInfinity.
A router may only prematurely age its own (self-originated) link state
advertisements. These are the link state advertisements having the
router's own OSPF Router ID in the Advertising Router field.
15. Virtual Links
The single backbone area (Area ID = 0) cannot be disconnected, or some
areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can be
configured through non-backbone areas. Virtual links serve to connect
separate components of the backbone. The two endpoints of a virtual
link are area border routers. The virtual link must be configured in
both routers. The configuration information in each router consists of
the other virtual endpoint (the other area border router), and the non-
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backbone area the two routers have in common (called the transit area).
Virtual links cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-point
network (belonging to the backbone) joining the two area border routers.
An attempt is made to establish an adjacency over the virtual link.
When this adjacency is established, the virtual link will be included in
backbone router links advertisements, and OSPF packets pertaining to the
backbone area will flow over the adjacency. Such an adjacency has been
referred to as a "virtual adjacency".
In each endpoint router, the cost and viability of the virtual link is
discovered by examining the routing table entry for the other endpoint
router. (The entry's associated area must be the configured transit
area). Actually, there may be a separate routing table entry for each
Type of Service. These are called the virtual link's corresponding
routing table entries. The Interface Up event occurs for a virtual link
when its corresponding TOS 0 routing table entry becomes reachable.
Conversely, the Interface Down event occurs when its TOS 0 routing table
entry becomes unreachable.[18] In other words, the virtual link's
viability is determined by the existence of an intra-area path, through
the transit area, between the two endpoints. The other details
concerning virtual links are as follows:
o AS external links are NEVER flooded over virtual adjacencies. This
would be duplication of effort, since the same AS external links are
already flooded throughout the virtual link's transit area. For
this same reason, AS external link advertisements are not summarized
over virtual adjacencies during the database exchange process.
o The cost of a virtual link is NOT configured. It is defined to be
the cost of the intra-area path between the two defining area border
routers. This cost appears in the virtual link's corresponding
routing table entry. When the cost of a virtual link changes, a new
router links advertisement should be originated for the backbone
area.
o Just as the virtual link's cost and viability are determined by the
routing table build process (through construction of the routing
table entry for the other endpoint), so are the IP interface address
for the virtual interface and the virtual neighbor's IP address.
These are used when sending protocol packets over the virtual link.
o In each endpoint's router links advertisement for the backbone, the
virtual link is represented as a link having link type 4, Link ID
set to the virtual neighbor's OSPF Router ID and Link Data set to
the virtual interface's IP address. See Section 12.4.1 for more
information. Also, it may be the case that there is a TOS 0 path,
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but no non-zero TOS paths to the other endpoint router. In this
case, non-zero TOS costs must be set to LSInfinity in the router
links advertisement.
o When virtual links are configured for the backbone, information
concerning backbone networks should not be condensed before being
summarized for the transit areas. In other words, each backbone
network should be advertised in a separate summary link
advertisement, regardless of the backbone's configured area address
ranges. See Section 12.4.3 for more information.
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be hard
to estimate for a virtual link. It is better to err on the side of
making it too large.
16. Calculation Of The Routing Table
This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At each
step, the router must access individual pieces of the link state
databases (e.g., a router links advertisement originated by a certain
router). This access is performed by the lookup function discussed in
Section 12.2. The lookup process may return a link state advertisement
whose LS age is equal to MaxAge. Such an advertisement should not be
used in the routing table calculation, and is treated just as if the
lookup process had failed.
The OSPF routing table's organization is explained in Section 11. Two
examples of the routing table build process are presented in Sections
11.2 and 11.3. This process can be broken into the following steps:
(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so that
changes in routing table entries can be identified.
(2) The intra-area routes are calculated by building the shortest path
tree for each attached area. In particular, all routing table
entries whose Destination type is "area border router" are
calculated in this step. This step is described in two parts. At
first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks are
incorporated into the tree.
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(3) The inter-area routes are calculated, through examination of summary
link advertisements. If the router is attached to multiple areas
(i.e., it is an area border router), only backbone summary link
advertisements are examined.
(4) For those routing entries whose next hop is over a virtual link, a
real (physical) next hop is calculated. The real next hop will be
on one of the router's directly attached networks. This step only
concerns routers having configured virtual links.
(5) Routes to external destinations are calculated, through examination
of AS external link advertisements. The location of the AS boundary
routers (which originate the AS external link advertisements) has
been determined in steps 2-4.
Steps 2-5 are explained in further detail below. The explanations
describe the calculations for TOS 0 only. It may also be necessary to
perform each step (separately) for each of the non-zero TOS values.[19]
For more information concerning the building of non-zero TOS routes see
Section 16.9.
Changes made to routing table entries as a result of these calculations
can cause the OSPF protocol to take further actions. For example, a
change to an intra-area route will cause an area border router to
originate new summary link advertisements (see Section 12.4). See
Section 16.7 for a complete list of the OSPF protocol actions resulting
from routing table table changes.
16.1 Calculating the shortest-path tree for an area
This calculation yields the set of intra-area routes associated with an
area (called hereafter Area A). A router calculates the shortest-path
tree using itself as the root.[20] The formation of the shortest path
tree is done here in two stages. In the first stage, only links between
routers and transit networks are considered. Using the Dijkstra
algorithm, a tree is formed from this subset of the link state database.
In the second stage, leaves are added to the tree by considering the
links to stub networks.
The procedure will be explained using the graph terminology that was
introduced in Section 2. The area's link state database is represented
as a directed graph. The graph's vertices are routers, transit networks
and stub networks. The first stage of the procedure concerns only the
transit vertices (routers and transit networks) and their connecting
links. Throughout the shortest path calculation, the following data is
also associated with each transit vertex:
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Vertex (node) ID
A 32-bit number uniquely identifying the vertex. For router
vertices this is the OSPF Router ID. For network vertices, this is
the IP address of the network's Designated Router.
A link state advertisement
Each transit vertex has an associated link state advertisement. For
router vertices, this is a router links advertisement. For transit
networks, this is a network links advertisement (which is actually
originated by the network's Designated Router). In any case, the
advertisement's Link State ID is always equal to the above Vertex
ID.
List of next hops
The list of next hops for the current shortest paths from the root
to this vertex. There can be multiple shortest paths due to the
equal-cost multipath capability. Each next hop indicates the
outgoing router interface to use when forwarding traffic to the
destination. On multi-access networks, the next hop also includes
the IP address of the next router (if any) in the path towards the
destination.
Distance from root
The link state cost of the current shortest path(s) from the root to
the vertex. The link state cost of a path is calculated as the sum
of the costs of the path's constituent links (as advertised in
router links and network links advertisements). One path is said to
be "shorter" than another if it has a smaller link state cost.
The first stage of the procedure can now be summarized as follows. At
each iteration of the algorithm, there is a list of candidate vertices.
The shortest paths from the root to these vertices have not
(necessarily) been found. The candidate vertex closest to the root is
added to the shortest-path tree, removed from the candidate list, and
its adjacent vertices are examined for possible addition to/modification
of the candidate list. The algorithm then iterates again. It
terminates when the candidate list becomes empty.
The following steps describe the first stage in detail. Remember that
we are computing the shortest path tree for Area A. All references to
link state database lookup below are from Area A's database.
(1) Initialize the algorithm's data structures. Clear the list of
candidate vertices. Initialize the shortest-path tree to only the
root (which is the router doing the calculation).
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(2) Call the vertex just added to the tree vertex V. Examine the link
state advertisement associated with vertex V. This is a lookup in
the area link state database based on the Vertex ID. Each link
described by the advertisement gives the cost to an adjacent vertex.
For each described link, (say it joins vertex V to vertex W):
(a) If this is a link to a stub network, examine the next link in
V's advertisement. Links to stub networks will be considered in
the second stage of the shortest path calculation.
(b) Otherwise, W is a transit vertex (router or transit network).
Look up the vertex W's link state advertisement (router links or
network links) in Area A's link state database. If the
advertisement does not exist, or its age is equal to MaxAge, or
it does not have a link back to vertex V, examine the next link
in V's advertisement. Both ends of a link must advertise the
link before it will be used for data traffic.[21]
(c) If vertex W is already on the shortest-path tree, examine the
next link in the advertisement.
(d) If the cost of the link (from V to W) is LSInfinity, the link
should not be used for data traffic. In this case, examine the
next link in the advertisement.
(e) Calculate the link state cost D of the resulting path from the
root to vertex W. D is equal to the sum of the link state cost
of the (already calculated) shortest path to vertex V and the
advertised cost of the link between vertices V and W. If D is:
o Greater than the value that already appears for vertex W on
the candidate list, then examine the next link.
o Equal to the value that appears for vertex W on the the
candidate list, calculate the set of next hops that result
from using the advertised link. Input to this calculation
is the destination (W), and its parent (V). This
calculation is shown in Section 16.1.1. This set of hops
should be added to the next hop values that appear for W on
the candidate list.
o Less than the value that appears for vertex W on the the
candidate list, or if W does not yet appear on the candidate
list, then set the entry for W on the candidate list to
indicate a distance of D from the root. Also calculate the
list of next hops that result from using the advertised
link, setting the next hop values for W accordingly. The
next hop calculation is described in Section 16.1.1; it
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takes as input the destination (W) and its parent (V).
(3) If at this step the candidate list is empty, the shortest-path tree
(of transit vertices) has been completely built and this stage of
the algorithm terminates. Otherwise, choose the vertex belonging to
the candidate list that is closest to the root, and add it to the
shortest-path tree (removing it from the candidate list in the
process).
(4) Possibly modify the routing table. For those routing table entries
modified, the associated area will be set to Area A, the path type
will be set to intra-area, and the cost will be set to the newly
discovered shortest path's calculated distance.
If the newly added vertex is an area border router, a routing table
entry is added whose destination type is "area border router". The
Options field found in the associated router links advertisement is
copied into the routing table entry's Optional capabilities field.
If the newly added vertex is an AS boundary router, the routing
table entry of type "AS boundary router" for the destination is
located. Since routers can belong to more than one area, it is
possible that several sets of intra-area paths exist to the AS
boundary router, each set using a different area. However, the AS
boundary router's routing table entry must indicate a set of paths
which utilize a single area. The area leading to the routing table
entry is selected as follows: A set of intra-area paths having no
virtual next hops is always preferred over a set of intra-area paths
in which some virtual next hops appear[22] ; all other things being
equal the set of paths having lower cost is preferred. Note that
whenever an AS boundary router's routing table entry is
added/modified, the Options found in the associated router links
advertisement is copied into the routing table entry's Optional
capabilities field.
If the newly added vertex is a transit network, the routing table
entry for the network is located. The entry's destination ID is the
IP network number, which can be obtained by masking the Vertex ID
(Link State ID) with its associated subnet mask (found in the
associated network links advertisement). If the routing table entry
already exists (i.e., there is already an intra-area route to the
destination installed in the routing table), multiple vertices have
mapped to the same IP network. For example, this can occur when a
new Designated Router is being established. In this case, the
current routing table entry should be overwritten if and only if the
newly found path is just as short and the current routing table
entry's Link State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement.
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If there is no routing table entry for the network (the usual case),
a routing table entry for the IP network should be added. The
routing table entry's Link State origin should be set to the newly
added vertex' link state advertisement.
(5) Iterate the algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's second stage.
In this stage, all router vertices are again examined. Those that have
been determined to be unreachable in the above first phase are
discarded. For each reachable router vertex (call it V), the associated
router links advertisement is found in the link state database. Each
stub network link appearing in the advertisement is then examined, and
the following steps are executed:
(1) If the cost of the stub network link is LSInfinity, the link should
not be used for data traffic. In this case, go on to examine the
next stub network link in the advertisement.
(2) Otherwise, Calculate the distance D of stub network from the root.
D is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's advertised
cost. Compare this distance to the current best cost to the stub
network. This is done by looking up the network's current routing
table entry. If the calculated distance D is larger, go on to
examine the next stub network link in the advertisement.
(3) If this step is reached, the stub network's routing table entry must
be updated. Calculate the set of next hops that would result from
using the stub network link. This calculation is shown in Section
16.1.1; input to this calculation is the destination (the stub
network) and the parent vertex (the router vertex). If the distance
D is the same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops. In
this case, the routing table already has a Link State origin. If
this Link State origin is a router links advertisement whose Link
State ID is smaller than V's Router ID, reset the Link State origin
to V's router links advertisement.
Otherwise D is smaller than the routing table cost. Overwrite the
current routing table entry by setting the routing table entry's
cost to D, and by setting the entry's list of next hops to the newly
calculated set. Set the routing table entry's Link State origin to
V's router links advertisement. Then go on to examine the next stub
network link.
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For all routing table entries added/modified in the second stage, the
associated area will be set to Area A and the path type will be set to
intra-area. When the list of reachable router links is exhausted, the
second stage is completed. At this time, all intra-area routes
associated with Area A have been determined.
The specification does not require that the above two stage method be
used to calculate the shortest path tree. However, if another algorithm
is used, an identical tree must be produced. For this reason, it is
important to note that links between transit vertices must be
bidirectional in ordered to be included in the above tree. It should
also be mentioned that algorithms exist for incrementally updating the
shortest-path tree (see [BBN]).
16.1.1 The next hop calculation
This section explains how to calculate the current set of next hops to
use for a destination. Each next hop consists of the outgoing interface
to use in forwarding packets to the destination together with the next
hop router (if any). The next hop calculation is invoked each time a
shorter path to the destination is discovered. This can happen in
either stage of the shortest-path tree calculation (see Section 16.1).
In stage 1 of the shortest-path tree calculation a shorter path is found
as the destination is added to the candidate list, or when the
destination's entry on the candidate list is modified (Step 2e of Stage
1). In stage 2 a shorter path is discovered each time the destination's
routing table entry is modified (Step 3 of Stage 2).
The set of next hops to use for the destination may be recalculated
several times during the shortest-path tree calculation, as shorter and
shorter paths are discovered. In the end, the destination's routing
table entry will always reflect the next hops resulting from the
absolute shortest path(s).
Input to the next hop calculation is a) the destination and b) its
parent in the current shortest path between the root (the calculating
router) and the destination. The parent is always a transit vertex
(i.e., always a router or a transit network).
If there is at least one intervening router in the current shortest path
between the destination and the root, the destination simply inherits
the set of next hops from the parent. Otherwise, there are two cases.
In the first case, the parent vertex is the root (the calculating router
itself). This means that the destination is either a directly connected
network or directly connected router. The next hop in this case is
simply the OSPF interface connecting to the network/router; no next hop
router is required.
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In the second case, the destination is a router, and its parent vertex
is a network. The list of next hops is then determined by examining the
destination's router links advertisement. For each link in the
advertisement that points back to the parent network, the link's Link
Data field provides the IP address of a next hop router. The outgoing
interface to use can then be derived from the next hop IP address (or it
can be inherited from the parent network).
16.2 Calculating the inter-area routes
The inter-area routes are calculated by examining summary link
advertisements. If the router has active attachments to multiple areas,
only backbone summary link advertisements are examined. Routers
attached to a single area examine that area's summary links. In either
case, the summary links examined below are all part of a single area's
link state database (call it Area A).
Summary link advertisements are originated by the area border routers.
Each summary link advertisement in Area A is considered in turn.
Remember that the destination described by a summary link advertisement
is either a network (type 3 summary link advertisements) or an AS
boundary router (type 4 summary link advertisements). For each summary
link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, then
examine the next advertisement.
(2) If the advertisement was originated by the calculating router
itself, examine the next advertisement.
(3) If the collection of destinations described by the summary link
falls into one of the router's configured area address ranges (see
Section 3.5) and the particular area address range is active, the
summary link should be ignored. Active means that there are one or
more reachable (by intra-area paths) networks contained in the area
range. In this case, all addresses in the area range are assumed to
be either reachable via intra-area paths, or else to be unreachable
by any other means.
(4) Else, call the destination described by the advertisement N, and the
area border originating the advertisement BR. Look up the routing
table entry for BR having A as its associated area. If no such
entry exists for router BR (i.e., BR is unreachable in Area A), do
nothing with this advertisement and consider the next in the list.
Else, this advertisement describes an inter-area path to destination
N, whose cost is the distance to BR plus the cost specified in the
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advertisement. Call the cost of this inter-area path IAC.
(5) Next, look up the routing table entry for the destination N. (The
entry's Destination type is either Network or AS boundary router.)
If no entry exists for N or if the entry's path type is "AS
external", install the inter-area path to N, with associated area A,
cost IAC, next hop equal to the list of next hops to router BR, and
advertising router equal to BR.
(6) Else, if the paths present in the table are intra-area paths, do
nothing with the advertisement (intra-area paths are always
preferred).
(7) Else, the paths present in the routing table are also inter-area
paths. Install the new path through BR if it is cheaper, overriding
the paths in the routing table. Otherwise, if the new path is the
same cost, add it to the list of paths that appear in the routing
table entry.
16.3 Resolving virtual next hops
This step is only necessary in area-border routers having configured
virtual links. In these routers, some of the routing table entries may
have virtual next hops. That is, one or more of the next hops installed
in Sections 16.1 and 16.2 may be over a virtual link. However, when
forwarding data traffic to a destination, the next hops must always be
on a directly attached network.
In this section, each virtual next hop is replaced by a real next hop.
In the process a new routing table distance is calculated that may be
smaller than the previously calculated distance. In this case, the list
of next hops is pruned so that only those giving rise to the new
shortest distance are included, and the routing table entry's distance
is updated accordingly.
______________________________________
(Figure not included in text version.)
Figure 17: Resolving virtual next hops
______________________________________
[Moy] [Page 124]RFC 1247 OSPF Version 2 July 1991
This resolution of virtual next hops is done only for Destination types
Network or AS Boundary router. Suppose that one of a routing table
entry's next hops is a virtual link. This is determined by the
following combination: the routing table entry's path type is either
intra-area or inter-area, the area associated with the routing table
entry must be the backbone, yet the next hop belongs to a different area
(the virtual link's transit area).
Let N be the above entry's destination, and A the virtual link's transit
area. The real next hop (and new distance) is calculated as follows.
Let D be a distance counter, and set the real next hop NH to null.
Then, look up all the summary link advertisements for N in area A's
database, performing the following steps for each advertisement:[23]
(1) Call the border router that originated the advertisement BR. If
there is no routing table entry for BR having A as associated area
(i.e., BR is unreachable through Area A), examine the next
advertisement.
(2) Else, let X be the distance to BR via Area A. If the cost
advertised by BR (call it Y) to the destination is LSInfinity,
examine the next summary link advertisement. Else, the cost to
destination N through area border router BR is X+Y.
(3) If next hop NH is null or X+Y is smaller is smaller than D, set D to
X+Y and set the next hop NH to the next hop specified in router BR's
routing table entry.
At this point, the real next hop NH should be set, and the distance D
calculated should be less than or equal to the cost originally specified
in destination N's routing table entry. This same calculation should be
done for all of N's virtual next hops, and then N's new cost set to the
minimum calculated distance, with the its new set of next hops that
combination of non-virtual and recalculated next hops that correspond to
this (possibly same as original) distance.
The resolving of virtual next hops may produce unexpected results.
After the virtual next hops are resolved, traffic that was originally
scheduled to go over the virtual link may instead take a different path
through the virtual link's transit area. In other words, virtual links
allow transit traffic to be forwarded through an area, but do not
dictate the precise path that the traffic will take.
As an example, consider the Autonomous System pictured in Figure 17.
There is a single non-backbone area (Area 1) that physically divides the
backbone into two separate pieces. To maintain connectivity of the
[Moy] [Page 125]RFC 1247 OSPF Version 2 July 1991
backbone, a virtual link has been configured between routers RT1 and
RT4. On the right side of the figure, network N1 belongs to the
backbone. The dotted lines indicate that there is a much shorter
intra-area backbone path between router RT5 and network N1 (cost 20)
than there is between router RT4 and network N1 (cost 100). Both router
RT4 and router RT5 will inject summary link advertisements for network
N1 into Area 1.
After the shortest-path tree has been calculated for the backbone,
router RT1 (one end of the virtual link) will have selected router RT4
as the virtual next hop for all data traffic destined for network N1.
However, since router RT5 is so much closer to network N1, all routers
internal to Area 1 (e.g., routers RT2 and RT3) will forward their
network N1 traffic towards router RT5, instead of RT4. And indeed,
after resolving the virtual next hop by the above calculation, router
RT1 will also forward network N1 traffic towards RT5. So, in this
example the virtual link enables network N1 traffic to be forwarded
through the transit Area 1, but the actual path the data traffic takes
does not follow the virtual link.
16.4 Calculating AS external routes
AS external routes are calculated by examining AS external link
advertisements. Each of the AS external link advertisements is
considered in turn. Most AS external advertisements describe routes to
specific IP destinations. An AS external advertisement can also
describe a default route for the Autonomous System (destination =
DefaultDestination). For each AS external link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, then
examine the next advertisement.
(2) If the advertisement was originated by the calculating router
itself, examine the next advertisement.
(3) Call the destination described by the advertisement N. Look up the
routing table entry for the AS boundary router (ASBR) that
originated the advertisement. If no entry exists for router ASBR
(i.e., ASBR is unreachable), do nothing with this advertisement and
consider the next in the list.
Else, this advertisement describes an AS external path to
destination N. Examine the forwarding address specified in the
external advertisement. This indicates the IP address to which
packets for the destination should be forwarded. If forwarding
address is set to 0.0.0.0, packets should be sent to the ASBR
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itself. Otherwise, look up the forwarding address in the routing
table.[24] An intra-area or inter-area path must exist to the
forwarding address. If no such path exists, do nothing with the
advertisement and consider the next in the list.
Call the routing table distance to the forwarding address X (when
the forwarding address is set to 0.0.0.0, this is the distance to
the ASBR itself), and the cost specified in the advertisement Y. X
is in terms of the link state metric, and Y is a Type 1 or 2
external metric.
(4) Next, look up the routing table entry for the destination N. If no
entry exists for N, install the AS external path to N, with next hop
equal to the list of next hops to the forwarding address, and
advertising router equal to ASBR. If the external metric type is 1,
then the path-type is set to type 1 external and the cost is equal
to X+Y. If the external metric type is 2, the the path-type is set
to type 2 external, the link state component of the route's cost is
X, and the Type 2 cost is Y.
(5) Else, if the paths present in the table are not type 1 or type 2
external paths, do nothing (AS external paths have the lowest
priority).
(6) Otherwise, compare the cost of this new AS external path to the ones
present in the table. Type 1 external paths are always shorter than
Type 2 external paths. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding address and the
advertised Type 1 metric (X+Y). Type 2 external paths are compared
by looking at the advertised Type 2 metrics, and then if necessary,
the distance to the forwarding addresses.
If the new path is shorter, it replaces the present paths in the
routing table entry. If the new path is the same cost, it is added
to the routing table entry's list of paths.
16.5 Incremental updates --- summary links
When a new summary link advertisement is received, it is not necessary
to recalculate the entire routing table. Call the destination described
by the summary link advertisement N, and let A be the area to which the
advertisement belongs.
Look up the routing table entry for N. If the next hop to N is a
virtual link through Area A (this means that the entry's associated area
is the backbone, and the listed next hop does not belong to the
backbone, but instead belongs to Area A), the real next hop must again
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be resolved. This means running the algorithm in Section 16.3 for
destination N only.
Else, if there is an intra-area route to destination N nothing need be
done (intra-area routes always take precedence). Otherwise, if Area A
is the router's sole attached area, or Area A is the backbone, the
procedure in Section 16.2 will have to be performed, but only for those
summary link advertisements whose destination is N. Before this
procedure is performed, the present routing table entry for N should be
invalidated (but kept for comparison purposes). If this procedure leads
to a virtual next hop, the algorithm in Section 16.3 will again have to
be performed in order to calculate the real next hop.
If N's routing table entry changes, and N is an AS boundary router, the
AS external links will have to be reexamined (Section 16.4).
16.6 Incremental updates --- AS external links
When a new AS external link advertisement is received, it is not
necessary to recalculate the entire routing table. Call the destination
described by the AS external link advertisement N. If there is already
an intra-area or inter-area route to the destination, no recalculation
is necessary (these routes take precedence).
Otherwise, the procedure in Section 16.4 will have to be performed, but
only for those AS external link advertisements whose destination is N.
Before this procedure is performed, the present routing table entry for
N should be invalidated.
16.7 Events generated as a result of routing table changes
Changes to routing table entries sometimes cause the OSPF area border
routers to take additional actions. These routers need to act on the
following routing table changes:
o The cost or path type of a routing table entry has changed. If the
destination described by this entry is a Network or AS boundary
router, and this is not simply a change of AS external routes, new
summary link advertisements may have to be generated (potentially
one for each attached area, including the backbone). See Section
12.4.3 for more information. If a previously advertised entry has
been deleted, or is no longer advertisable to a particular area, the
advertisement must be flushed from the routing domain by setting its
age to MaxAge and reflooding (see Section 14.1).
[Moy] [Page 128]RFC 1247 OSPF Version 2 July 1991
o A routing table entry associated with a configured virtual link has
changed. The destination of such a routing table entry is an area
border router. The change indicates a modification to the virtual
link's cost or viability.
If the entry indicates that the area border router is newly
reachable (via TOS 0), the corresponding virtual link is now
operational. An Interface Up event should be generated for the
virtual link, which will cause a virtual adjacency to begin to form
(see Section 10.3). At this time the virtual interface's IP address
and the virtual neighbor's IP address are also calculated.
If the entry indicates that the area border router is no longer
reachable (via TOS 0), the virtual link and its associated adjacency
should be destroyed. This means an Interface Down event should be
generated for the associated virtual link.
If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router links advertisement for
the backbone must be originated. This in turn may cause further
routing table changes.
16.8 Equal-cost multipath
The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to calculate the
routing table, and in the definition of the routing table structure.
Each one of the multiple routes will be of the same type (intra-area,
inter-area, type 1 external or type 2 external), cost, and will have the
same associated area. However, each route specifies a separate next hop
and advertising router.
There is no requirement that a router running OSPF keep track of all
possible equal-cost routes to a destination. An implementation may
choose to keep only a fixed number of routes to any given destination.
This does not affect any of the algorithms presented in this
specification.
16.9 Building the non-zero-TOS portion of the routing table
The OSPF protocol can calculate a different set of routes for each IP
TOS (see Section 2.4). Support for TOS-based routing is optional.
TOS-capable and non-TOS-capable routers can be mixed in an OSPF routing
domain. Routers not supporting TOS calculate only the TOS 0 route to
each destination. These routes are then used to forward all data
[Moy] [Page 129]RFC 1247 OSPF Version 2 July 1991
traffic, regardless of the TOS indications in the data packet's IP
header. A router that does not support TOS indicates this fact to the
other OSPF routers by clearing the T-bit in the Options field of its
router links advertisement.
The above sections detailing the routing table calculations handle the
TOS 0 case only. In general, for routers supporting TOS-based routing,
each piece of the routing table calculation must be rerun separately for
the non-zero TOS values. When calculating routes for TOS X, only TOS X
metrics can be used. Any link state advertisement may specify a
separate cost for each TOS (a cost for TOS 0 must always be specified).
The encoding of TOS in OSPF link state advertisements is described in
Section 12.3.
An advertisement can specify that it is restricted to TOS 0 (i.e., non-
zero TOS is not handled) by clearing the T-bit in the link state
advertisement's Option field. Such advertisements are not used when
calculating routes for non-zero TOS. For this reason, it is possible
that a destination is unreachable for some non-zero TOS. In this case,
the TOS 0 path is used when forwarding packets (see Section 11.1).
The following lists the modifications needed when running the routing
table calculation for a non-zero TOS value (called TOS X). In general,
routers and advertisements that do not support TOS are omitted from the
calculation.
Calculating the shortest-path tree (Section 16.1).
Routers that do not support TOS-based routing should be omitted from
the shortest-path tree calculation. These routers are identified as
those having the T-bit reset in their router links advertisements.
Such routers should never be added to the Dijktra algorithm's
candidate list, nor should their router links advertisements be
examined when adding the stub networks to the tree.
Calculating the inter-area routes (Section 16.2).
Inter-area paths are the concatenation of a path to an area border
router with a summary link. When calculating TOS X routes, both
path components must also specify TOS X. In other words, only TOS X
paths to the area border router are examined, and the area border
router must be advertising a TOS X route to the destination. Note
that this means that summary link advertisements having the T-bit
reset in their Options field are not considered.
Resolving virtual next hops (Section 16.3).
This calculation again considers the concatenation of a path to an
area border router with a summary link. As with inter-area routes,
only TOS X paths to the area border router are examined, and the
[Moy] [Page 130]RFC 1247 OSPF Version 2 July 1991
area border router must be advertising a TOS X route to the
destination.
Calculating AS external routes (Section 16.4).
This calculation considers the concatenation of a path to a
forwarding address with an AS external link. Only TOS X paths to
the forwarding address are examined, and the AS boundary router must
be advertising a TOS X route to the destination. Note that this
means that AS external link advertisements having the T-bit reset in
their Options field are not considered.
In addition, the advertising AS boundary router must also be
reachable for its advertisements to be considered (see Section
16.4). However, if the advertising router and the forwarding
address are not one in the same, the advertising router need only be
reachable via TOS 0.
[Moy] [Page 131]RFC 1247 OSPF Version 2 July 1991
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router links advertisement as a host route.
[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.
[4]Note that no host route is generated for, and no IP packets can
be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state.
[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the the Backup Designated Router RT2. If router RT1
crashes (or maybe its interface to the network dies), the other
routers on the network will detect RT1's absence within
RouterDeadInterval seconds. All routers may not detect this at
precisely the same time; the routers that detect RT1's absence
before RT2 does will, for a time, select RT2 to be both Designated
Router and Backup Designated Router. When RT2 detects that RT1 is
gone it will move itself to Designated Router. At this time, the
remaining router having highest Router Priority will be selected as
Backup Designated Router.
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors
is bidirectional, and that each of the neighbors has a functioning
routing protocol layer.
[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a
Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.
[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
process a Seq Number Mismatch event, and therefore to also go back
to ExStart state.
[Moy] [Page 132]RFC 1247 OSPF Version 2 July 1991
[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.
[10]It is assumed that, for two different address ranges matching
the destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this
assumption. Such subnet mask configurations cannot be handled by the
OSPF protocol.
[11]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
link state instance as it is flooded throughout the routing domain.
If two advertisements differ by more than this, they are assumed to
be different instances of the same advertisement. This can occur
when a router restarts and loses track of its previous sequence
number. See Section 13.4 for more details.
[12]When two advertisements have different checksums, they are
assumed to be separate instances. This can occur when a router
restarts, and loses track of its previous sequence number. In this
case, since the two advertisements have the same sequence number, it
is not possible to determine which link state is actually newer. If
the wrong advertisement is accepted as newer, the originating router
will originate another instance. See Section 13.4 for further
details.
[13]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network links advertisement must be found based solely on its
Link State ID. The lookup in this case is still well defined, since
no two network advertisements can have the same Link State ID.
[14]This clause covers the case: Inter-area routes are not
summarized to the backbone. This is because inter-area routes are
always associated with the backbone area.
[15]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree only for
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of the shortest path
tree [BBN]. These algorithms are beyond the scope of this
specification.
[16]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.
[Moy] [Page 133]RFC 1247 OSPF Version 2 July 1991
[17]It should be a relatively rare occurrence for an advertisement's
age to reach MaxAge. Usually, the advertisement will be replaced by
a more recent instance before it ages out.
[18]Only the TOS 0 routes are important here. This is because all
routing protocol packets are sent with TOS= 0. See Appendix A.
[19]It may be the case that paths to certain destinations do not
vary based on TOS. For these destinations, the routing calculation
need not be repeated for each TOS value. In addition, there need
only be a single routing table entry for these destinations (instead
of a separate entry for each TOS value).
[20]Strictly speaking, because of equal-cost multipath, the
algorithm does not create a tree. We continue to use the "tree"
terminology because that is what occurs most often in the existing
literature.
[21]This means that before data traffic will flow between a pair of
neighboring routers, their link state databases must be
synchronized. Before synchronization (neighbor state < Full), a
router will not include the connection to its neighbor in its link
state advertisements.
[22]As a result of this clause, when a virtual link exists between
the calculating router and an AS boundary router, the intra-area
path through the virtual link's transit area is always preferred
over the virtual link itself.
[23]Note the similarity between this procedure and the calculation
of inter-area routes by a router internal to Area A.
[24]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.4
for more details.
[Moy] [Page 134]RFC 1247 OSPF Version 2 July 1991
References
[BBN] McQuillan, J.M., Richer, I. and Rosen, E.C. ARPANET
Routing Algorithm Improvements. BBN Technical Report 3803,
April 1978.
[DEC] Digital Equipment Corporation. Information processing
systems -- Data communications -- Intermediate System to
Intermediate System Intra-Domain Routing Protocol. October
1987.
[McQuillan] McQuillan, J. et.al. The New Routing Algorithm for the
Arpanet. IEEE Transactions on Communications, May 1980.
[Perlman] Perlman, Radia. Fault-Tolerant Broadcast of Routing
Information. Computer Networks, Dec. 1983.
[RFC 791] Postel, Jon. Internet Protocol. September 1981
[RFC 944] ANSI X3S3.3 86-60. Final Text of DIS 8473, Protocol for
Providing the Connectionless-mode Network Service. March
1986.
[RFC 1060] Reynolds, J. and Postel, J. Assigned Numbers. March 1990.
[RFC 1112] Deering, S.E. Host extensions for IP multicasting. May
1988.
[RFC 1131] Moy, J. The OSPF Specification. October 1989.
[RS-85-153] Leiner, Dr. Barry M., et.al. The DARPA Internet Protocol
Suite. DDN Protocol Handbook, April 1985.
[Moy] [Page 135]RFC 1247 OSPF Version 2 July 1991
A. OSPF data formats
This appendix describes the format of OSPF protocol packets and OSPF
link state advertisements. The OSPF protocol runs directly over the IP
network layer. Before any data formats are described, the details of
the OSPF encapsulation are explained.
Next the OSPF options field is described. This field describes various
capabilities that may or may not be supported by pieces of the OSPF
routing domain. It is contained both in OSPF protocol packets and in
OSPF link state advertisements.
OSPF packet formats are detailed in Section A.3. A description of OSPF
link state advertisements appears in Section A.4.
A.1 Encapsulation of OSPF packets
OSPF runs directly over the Internet Protocol's network layer. OSPF
packets are therefore encapsulated solely by IP and local network
headers.
OSPF does not define a way to fragment its protocol packets, and depends
on IP fragmentation when transmitting packets larger than the network
MTU. The OSPF packet types that are likely to be large (Database
Description Packets, Link State Request, Link State Update, and Link
State Acknowledgment packets) can usually be split into several separate
protocol packets, without loss of functionality. This is recommended;
IP fragmentation should be avoided whenever possible. Using this
reasoning, an attempt should be made to limit the sizes of packets sent
over virtual links to 576 bytes. However, if necessary, the length of
OSPF packets can be up to 65,535 bytes (including the IP header).
The other important features of OSPF's IP encapsulation are:
o Use of IP multicast. Some OSPF messages are multicast, when sent
over multi-access networks. Two distinct IP multicast addresses are
used. Packets destined to these multicast addresses should never be
forwarded. Such packets are meant to travel a single hop only. To
ensure that these packets will not travel multiple hops, their IP
TTL must be set to 1.
AllSPFRouters
This multicast address has been assigned the value 224.0.0.5.
All routers running OSPF should be prepared to receive packets
sent to this address. Hello packets are always sent to this
destination. Also, certain protocol packets are sent to this
[Moy] [Page 136]RFC 1247 OSPF Version 2 July 1991
address during the flooding procedure.
AllDRouters
This multicast address has been assigned the value 224.0.0.6.
Both the Designated Router and Backup Designated Router must be
prepared to receive packets destined to this address. Certain
packets are sent to this address during the flooding procedure.
o OSPF is IP protocol number 89. This number has been registered with
the Network Information Center. IP protocol number assignments are
documented in [RFC 1060].
o Routing protocol packets are sent with IP TOS of 0. The OSPF
protocol supports TOS-based routing. Routes to any particular
destination may vary based on TOS. However, all OSPF routing
protocol packets are sent with the DTR bits in the IP header's TOS
field (see [RFC 791]) set to 0.
o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [RFC 791] may help implement this objective.
[Moy] [Page 137]RFC 1247 OSPF Version 2 July 1991
A.2 The options field
The OSPF options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. The options
field enables OSPF routers to support (or not support) optional
capabilities, and to communicate their capability level to other OSPF
routers. Through this mechanism routers of differing capabilities can
be mixed within an OSPF routing domain.
When used in Hello packets, the options field allows a router to reject
a neighbor because of a capability mismatch. Alternatively, when
capabilities are exchanged in Database Description packets a router can
choose not to forward certain LSA types to a neighbor because of its
reduced functionality. Lastly, listing capabilities in LSAs allows
routers to route traffic around reduced functionality routers, by
excluding them from parts of the routing table calculation.
Two capabilities are currently defined. For each capability, the effect
of the capability's appearance (or lack of appearance) in Hello packets,
Database Description packets and link state advertisements is specified
below. For example, the external routing capability (below called the
E-bit) has meaning only in OSPF Hello Packets. Routers should reset
(i.e. clear) the unassigned part of the capability field when sending
Hello packets or Database Description packets and when originating link
state advertisements.
Additional capabilities may be assigned in the future. Routers
encountering unrecognized capabilities in received Hello Packets,
Database Description packets or link state advertisements should ignore
the capability and process the packet/advertisement normally.
+-+-+-+-+-+-+-+-+
| | | | | | |E|T|
+-+-+-+-+-+-+-+-+
The options field
T-bit
This describes the router's TOS capability. If the T-bit is reset,
then the router supports only a single TOS (TOS 0). Such a router
is also said to be incapable of TOS-routing. The absence of the T-
bit in a router links advertisement causes the router to be skipped
when building a non-zero TOS shortest-path tree (see Section 16.9).
In other words, routers incapable of TOS routing will be avoided as
much as possible when forwarding data traffic requesting a non-zero
TOS. The absence of the T-bit in a summary link advertisement or an
AS external link advertisement indicates that the advertisement is
[Moy] [Page 138]RFC 1247 OSPF Version 2 July 1991
describing a TOS 0 route only (and not routes for non-zero TOS).
E-bit
AS external link advertisements are not flooded into/through OSPF
stub areas (see Section 3.6). The E-bit ensures that all members of
a stub area agree on that area's configuration. The E-bit is
meaningful only in OSPF Hello packets. When the E-bit is reset in
the Hello packet sent out a particular interface, it means that the
router will neither send nor receive AS external link state
advertisements on that interface (in other words, the interface
connects to a stub area). Two routers will not become neighbors
unless they agree on the state of the E-bit.
[Moy] [Page 139]RFC 1247 OSPF Version 2 July 1991
A.3 OSPF Packet Formats
There are five distinct OSPF packet types. All OSPF packet types begin
with a standard 24 byte header. This header is described first. Each
packet type is then described in a succeeding section. In these
sections each packet's division into fields is displayed, and then the
field definitions are enumerated.
All OSPF packet types (other than the OSPF Hello packets) deal with
lists of link state advertisements. For example, Link State Update
packets implement the flooding of advertisements throughout the OSPF
routing domain. Because of this, OSPF protocol packets cannot be parsed
unless the format of link state advertisements is also understood. The
format of Link state advertisements is described in Section A.4.
The receive processing of OSPF packets is detailed in Section 8.2. The
sending of OSPF packets is explained in Section 8.1.
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RFC 1247 OSPF Version 2 July 1991
A.3.1 The OSPF packet header
Every OSPF packet starts with a common 24 byte header. This header
contains all the necessary information to determine whether the packet
should be accepted for further processing. This determination is
described in Section 8.2 of the specification.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | Type | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version #
The OSPF version number. This specification documents version 2 of
the protocol.
Type
The OSPF packet types are as follows. The format of each of these
packet types is described in a succeeding section.
Type Description
________________________________
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
[Moy] [Page 141]RFC 1247 OSPF Version 2 July 1991
Packet length
The length of the protocol packet in bytes. This length includes
the standard OSPF header.
Router ID
The Router ID of the packet's source. In OSPF, the source and
destination of a routing protocol packet are the two ends of an
(potential) adjacency.
Area ID
A 32 bit number identifying the area that this packet belongs to.
All OSPF packets are associated with a single area. Most travel a
single hop only. Packets travelling over a virtual link are
labelled with the backbone area ID of 0.
Checksum
The standard IP checksum of the entire contents of the packet,
excluding the 64-bit authentication field. This checksum is
calculated as the 16-bit one's complement of the one's complement
sum of all the 16-bit words in the packet, excepting the
authentication field. If the packet's length is not an integral
number of 16-bit words, the packet is padded with a byte of zero
before checksumming.
AuType
Identifies the authentication scheme to be used for the packet.
Authentication is discussed in Appendix E of the specification.
Consult Appendix E for a list of the currently defined
authentication types.
Authentication
A 64-bit field for use by the authentication scheme.
[Moy] [Page 142]RFC 1247 OSPF Version 2 July 1991
A.3.2 The Hello packet
Hello packets are OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hellos are
multicast on those physical networks having a multicast or broadcast
capability, enabling dynamic discovery of neighboring routers.
All routers connected to a common network must agree on certain
parameters (network mask, hello and dead intervals). These parameters
are included in Hello packets, so that differences can inhibit the
forming of neighbor relationships. A detailed explanation of the
receive processing for Hello packets is presented in Section 10.5. The
sending of Hello packets is covered in Section 9.5.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 1 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| HelloInt | Options | Rtr Pri |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DeadInt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Backup Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
[Moy] [Page 143]RFC 1247 OSPF Version 2 July 1991
Network mask
The network mask associated with this interface. For example, if
the interface is to a class B network whose third byte is used for
subnetting, the network mask is 0xffffff00.
Options
The optional capabilities supported by the router, as documented in
Section A.2.
HelloInt
The number of seconds between this router's Hello packets.
Rtr Pri
This router's Router Priority. Used in (Backup) Designated Router
election. If set to 0, the router will be ineligible to become
(Backup) Designated Router.
Deadint
The number of seconds before declaring a silent router down.
Designated Router
The identity of the Designated Router for this network, in the view
of the advertising router. The Designated Router is identified here
by its IP interface address on the network. Set to 0 if there is no
Designated Router.
Backup Designated Router
The identity of the Backup Designated Router for this network, in
the view of the advertising router. The Backup Designated Router is
identified here by its IP interface address on the network. Set to
0 if there is no backup Designated Router.
Neighbor
The Router IDs of each router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
DeadInt seconds.
[Moy] [Page 144]RFC 1247 OSPF Version 2 July 1991
A.3.3 The Database Description packet
Database Description packets are OSPF packet type 2. These packets are
exchanged when an adjacency is being initialized. They describe the
contents of the topological database. Multiple packets may be used to
describe the database. For this purpose a poll-response procedure is
used. One of the routers is designated to be master, the other a slave.
The master sends Database Description packets (polls) which are
acknowledged by Database Description packets sent by the slave
(responses). The responses are linked to the polls via the packets'
sequence numbers.
The format of the Database Description packet is very similar to both
the Link State Request and Link State Acknowledgment packets. The main
part of all three is a list of items, each item describing a piece of
the topological database. The sending of Database Description Packets
is documented in Section 10.8. The reception of Database Description
packets is documented in Section 10.6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 2 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 0 | Options |0|0|0|0|0|I|M|MS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DD sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| A |
+- Link State Advertisement -+
| Header |
+- -+
| |
+- -+
| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
0 These fields are reserved. They must be 0.
Options
The optional capabilities supported by the router, as documented in
Section A.2.
I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of database descriptions.
M-bit
The More bit. When set to 1, it indicates that more database
descriptions are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the router
is the master during the database exchange process. Otherwise, the
router is the slave.
DD sequence number
Used to sequence the collection of database description packets.
The initial value (indicated by the Init bit being set) should be
unique. The sequence number then increments until the complete
database description has been sent.
The rest of the packet consists of a (possibly partial) list of the
topological database's pieces. Each link state advertisement in the
database is described by its link state header. The link state header
is documented in Section A.4.1. It contains all the information
required to uniquely identify both the advertisement and the
advertisement's current instance.
[Moy] [Page 146]RFC 1247 OSPF Version 2 July 1991
A.3.4 The Link State Request packet
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its topological database are out of date. The Link
State Request packet is used to request the pieces of the neighbor's
database that are more up to date. Multiple Link State Request packets
may need to be used. The sending of Link State Request packets is the
last step in bringing up an adjacency.
A router that sends a Link State Request packet has in mind the precise
instance of the database pieces it is requesting (defined by LS sequence
number, LS checksum, and LS age). It may receive even more recent
instances in response.
The sending of Link State Request packets is documented in Section 10.9.
The reception of Link State Request packets is documented in Section
10.7.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 3 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Each advertisement requested is specified by its LS type, Link State ID,
and Advertising Router. This uniquely identifies the advertisement, but
not its instance. Link State Request packets are understood to be
requests for the most recent instance (whatever that might be).
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A.3.5 The Link State Update packet
Link State Update packets are OSPF packet type 4. These packets
implement the flooding of link state advertisements. Each Link State
Update packet carries a collection of link state advertisements one hop
further from its origin. Several link state advertisements may be
included in a single packet.
Link State Update packets are multicast on those physical networks that
support multicast/broadcast. In order to make the flooding procedure
reliable, flooded advertisements are acknowledged in Link State
Acknowledgment packets. If retransmission of certain advertisements is
necessary, the retransmitted advertisements are always carried by
unicast Link State Update packets. For more information on the reliable
flooding of link state advertisements, consult Section 13.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 4 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # advertisements |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- +-+
| Link state advertisements |
+- +-+
| ... |
# advertisements
The number of link state advertisements included in this update.
The body of the Link State Update packet consists of a list of link
state advertisements. Each advertisement begins with a common 20 byte
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header, the link state advertisement header. This header is described
in Section A.4.1. Otherwise, the format of each of the five types of
link state advertisements is different. Their formats are described in
Section A.4.
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RFC 1247 OSPF Version 2 July 1991
A.3.6 The Link State Acknowledgment packet
Link State Acknowledgment Packets are OSPF packet type 5. To make the
flooding of link state advertisements reliable, flooded advertisements
are explicitly acknowledged. This acknowledgment is accomplished
through the sending and receiving of Link State Acknowledgment packets.
Multiple link state advertisements can be acknowledged in a single
packet.
Depending on the state of the sending interface and the source of the
advertisements being acknowledged, a Link State Acknowledgment packet is
sent either to the multicast address AllSPFRouters, to the multicast
address AllDRouters, or as a unicast. The sending of Link State
Acknowledgement packets is documented in Section 13.5. The reception of
Link State Acknowledgement packets is documented in Section 13.7.
The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of link state
advertisement headers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version # | 5 | Packet length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Router ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Autype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- -+
| A |
+- Link State Advertisement -+
| Header |
+- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
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Each acknowledged link state advertisement is described by its link
state header. The link state header is documented in Section A.4.1. It
contains all the information required to uniquely identify both the
advertisement and the advertisement's current instance.
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RFC 1247 OSPF Version 2 July 1991
A.4 Link state advertisement formats
There are five distinct types of link state advertisements. Each link
state advertisement begins with a standard 20-byte link state header.
This header is explained in Section A.4.1. Succeeding sections then
diagram the separate link state advertisement types.
Each link state advertisement describes a piece of the OSPF routing
domain. Every router originates a router links advertisement. In
addition, whenever the router is elected Designated Router, it
originates a network links advertisement. Other types of link state
advertisements may also be originated (see Section 12.4). All link
state advertisements are then flooded throughout the OSPF routing
domain. The flooding algorithm is reliable, ensuring that all routers
have the same collection of link state advertisements. (See Section 13
for more information concerning the flooding algorithm). This
collection of advertisements is called the link state (or topological)
database.
From the link state database, each router constructs a shortest path
tree with itself as root. This yields a routing table (see Section 11).
For the details of the routing table build process, see Section 16.
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RFC 1247 OSPF Version 2 July 1991
A.4.1 The Link State Advertisement header
All link state advertisements begin with a common 20 byte header. This
header contains enough information to uniquely identify the
advertisement (LS type, Link State ID, and Advertising Router).
Multiple instances of the link state advertisement may exist in the
routing domain at the same time. It is then necessary to determine
which instance is more recent. This is accomplished by examining the LS
age, LS sequence number and LS checksum fields that are also contained
in the link state advertisement header.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age
The time in seconds since the link state advertisement was
originated.
Options
The optional capabilities supported by the described portion of the
routing domain. OSPF's optional capabilities are documented in
Section A.2.
LS type
The type of the link state advertisement. Each link state type has
a separate advertisement format. The link state types are as
follows (see Section 12.1.3 for further explanation):
[Moy] [Page 153]RFC 1247 OSPF Version 2 July 1991
LS Type Description
___________________________________
1 Router links
2 Network links
3 Summary link (IP network)
4 Summary link (ASBR)
5 AS external link
Link State ID
This field identifies the portion of the internet environment that
is being described by the advertisement. The contents of this field
depend on the advertisement's LS type. For example, in network
links advertisements the Link State ID is set to the IP interface
address of the network's Designated Router (from which the network's
IP address can be derived). The Link State ID is further discussed
in Section 12.1.4.
Advertising Router
The Router ID of the router that originated the link state
advertisement. For example, in network links advertisements this
field is set to the Router ID of the network's Designated Router.
LS sequence number
Detects old or duplicate link state advertisements. Successive
instances of a link state advertisement are given successive LS
sequence numbers. See Section 12.1.6 for more details.
LS checksum
The Fletcher checksum of the complete contents of the link state
advertisement. See Section 12.1.7 for more details.
length
The length in bytes of the link state advertisement. This includes
the 20 byte link state header.
[Moy] [Page 154]RFC 1247 OSPF Version 2 July 1991
A.4.2 Router links advertisements
Router links advertisements are the Type 1 link state advertisements.
Each router in an area originates a router links advertisement. The
advertisement describes the state and cost of the router's links (or
interfaces) to the area. All of the router's links to the area must be
described in a single router links advertisement. For details
concerning the construction of router links advertisements, see Section
12.4.1.
In router links advertisements, the Link State ID field is set to the
router's OSPF Router ID. The T-bit is set in the advertisement's Option
field if and only if the router is able to calculate a separate set of
routes for each IP TOS. Router links advertisements are flooded
throughout a single area only.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS | TOS 0 metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| ... |
bit E
When set, the router is an AS boundary router (E is for external)
bit B
When set, the router is an area border router (B is for border)
# links
The number of router links described by this advertisement. This
must be the total collection of router links to the area.
The following fields are used to describe each router link. Each router
link is typed (see the below Type field). The type field indicates the
kind of link being described. It may be a link to a transit network, to
another router or to a stub network. The values of all the other fields
describing a router link depend on the link's type. For example, each
link has an associated 32-bit data field. For links to stub networks
this field specifies the network's IP address mask. For the other link
types the Link Data specifies the router's associated IP interface
address.
Type
A quick description of the router link. One of the following. Note
that host routes are classified as links to stub networks whose
network mask is 0xffffffff.
Type Description
__________________________________________________
1 Point-to-point connection to another router
2 Connection to a transit network
3 Connection to a stub network
4 Virtual link
Link ID
Identifies the object that this router link connects to. Value
depends on the link's type. When connecting to an object that also
originates a link state advertisement (i.e., another router or a
transit network) the Link ID is equal to the other advertisement's
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Link State ID. This provides the key for looking up said
advertisement in the link state database. See Section 12.2 for more
details.
Type Link ID
______________________________________
1 Neighboring router's ID
2 IP address of Designated Router
3 IP network/subnet number
4 Neighboring router's ID
Link Data
Contents again depend on the link's Type field. For connections to
stub network, it specifies the network mask. For the other link
types it specifies the router's associated IP interface address.
This latter piece of information is needed during the routing table
build process, when calculating the IP address of the next hop. See
Section 16.1.1 for more details.
#metrics
The number of different TOS metrics given for this link, not
counting the required metric for TOS 0. For example, if no
additional TOS metrics are given, this field should be set to 0.
TOS 0 metric
The cost of using this router link for TOS 0.
For each link, separate metrics may be specified for each Type of
Service (TOS). The metric for TOS 0 must always be included, and was
discussed above. Metrics for non-zero TOS are described below. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for non-zero TOS values that are not
specified defaults to the TOS 0 cost. Metrics must be listed in order
of increasing TOS encoding. For example, the metric for TOS 16 must
always follow the metric for TOS 8 when both are specified.
TOS IP type of service that this metric refers to. The encoding of TOS
in OSPF link state advertisements is described in Section 12.3.
metric
The cost of using this outbound router link, for traffic of the
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specified TOS.
[Moy] [Page 158]RFC 1247 OSPF Version 2 July 1991
A.4.3 Network links advertisements
Network links advertisements are the Type 2 link state advertisements.
A network links advertisement is originated for each transit network in
the area. A transit network is a multi-access network that has more
than one attached router. The network links advertisement is originated
by the network's Designated Router. The advertisement describes all
routers attached to the network, including the Designated Router itself.
The advertisement's Link State ID field lists the IP interface address
of the Designated Router.
The distance from the network to all attached routers is zero, for all
types of service. This is why the TOS and metric fields need not be
specified in the network links advertisement. For details concerning
the construction of network links advertisements, see Section 12.4.2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network Mask
The IP address mask for the network. For example, a class A network
would have the mask 0xff000000.
Attached Router
The Router IDs of each of the routers attached to the network.
Actually, only those routers that are fully adjacent to the
Designated Router are listed. The Designated Router includes itself
in this list. The number of routers included can be deduced from
the link state advertisement's length field.
[Moy] [Page 159]RFC 1247 OSPF Version 2 July 1991
A.4.4 Summary link advertisements
Summary link advertisements are the Type 3 and 4 link state
advertisements. These advertisements are originated by area border
routers. A separate summary link advertisement is made for each
destination (known to the router) which belongs to the AS, yet is
outside the area. For details concerning the construction of summary
link advertisements, see Section 12.4.3.
Type 3 link state advertisements are used when the destination is an IP
network. In this case the advertisement's Link State ID field is an IP
network number. When the destination is an AS boundary router, a Type 4
advertisement is used, and the Link State ID field is the AS boundary
router's OSPF Router ID. (To see why it is necessary to advertise the
location of each ASBR, consult Section 16.4.) Other than the difference
in the Link State ID field, the format of Type 3 and 4 link state
advertisements is identical.
For stub areas, type 3 summary link advertisements can also be used to
describe a (per-area) default route. Default summary routes are used in
stub areas instead of flooding a complete set of external routes. When
describing a default summary route, the advertisement's Link State ID is
always set to DefaultDestination (0.0.0.0) and the Network Mask is set
to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for TOS 0 must be included, and is
always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the advertisement.
Otherwise, routes for the other TOS values are also described; if a cost
for a certain TOS is not included, its cost defaults to that specified
for TOS 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[Moy] [Page 160]RFC 1247 OSPF Version 2 July 1991
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network Mask
For Type 3 link state advertisements, this indicates the
destination's IP network mask. For example, when advertising the
location of a class A network the value 0xff000000 would be used.
This field is not meaningful and must be zero for Type 4 link state
advertisements.
For each specified type of service, the following fields are defined.
The number of TOS routes included can be calculated from the link state
advertisement's length field. Values for TOS 0 must be specified; they
are listed first. Other values must be listed in order of increasing
TOS encoding. For example, the cost for TOS 16 must always follow the
cost for TOS 8 when both are specified.
TOS The Type of Service that the following cost concerns. The encoding
of TOS in OSPF link state advertisements is described in Section
12.3.
metric
The cost of this route. Expressed in the same units as the
interface costs in the router links advertisements.
[Moy] [Page 161]RFC 1247 OSPF Version 2 July 1991
A.4.5 AS external link advertisements
AS external link advertisements are the Type 5 link state
advertisements. These advertisements are originated by AS boundary
routers. A separate advertisement is made for each destination (known
to the router) which is external to the AS. For details concerning the
construction of AS external link advertisements, see Section 12.4.3.
AS external link advertisements usually describe a particular external
destination. For these advertisements the Link State ID field specifies
an IP network number. AS external link advertisements are also used to
describe a default route. Default routes are used when no specific
route exists to the destination. When describing a default route, the
Link State ID is always set to DefaultDestination (0.0.0.0) and the
Network Mask is set to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3. Note that the cost for TOS 0 must be included, and is
always listed first. If the T-bit is reset in the advertisement's
Option field, only a route for TOS 0 is described by the advertisement.
Otherwise, routes for the other TOS values are also described; if a cost
for a certain TOS is not included, its cost defaults to that specified
for TOS 0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| TOS | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
[Moy] [Page 162]RFC 1247 OSPF Version 2 July 1991
Network Mask
The IP network mask for the advertised destination. For example,
when advertising a class A network the mask 0xff000000 would be
used.
For each specified type of service, the following fields are defined.
The number of TOS routes included can be calculated from the link state
advertisement's length field. Values for TOS 0 must be specified; they
are listed first. Other values must be listed in order of increasing
TOS encoding. For example, the cost for TOS 16 must always follow the
cost for TOS 8 when both are specified.
bit E
The type of external metric. If bit E is set, the metric specified
is a Type 2 external metric. This means the metric is considered
larger than any link state path. If bit E is zero, the specified
metric is a Type 1 external metric. This means that is is
comparable directly (without translation) to the link state metric.
Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the Forwarding address is set to 0.0.0.0, data
traffic will be forwarded instead to the advertisement's originator
(i.e., the responsible AS boundary router).
TOS The Type of Service that the following cost concerns. The encoding
of TOS in OSPF link state advertisements is described in Section
12.3.
metric
The cost of this route. Interpretation depends on the external type
indication (bit E above).
External Route Tag
A 32-bit field attached to each external route. This is not used by
the OSPF protocol itself. It may be used to communicate information
between AS boundary routers; the precise nature of such information
is outside the scope of this specification.
[Moy] [Page 163]RFC 1247 OSPF Version 2 July 1991
B. Architectural Constants
Several OSPF protocol parameters have fixed architectural values. These
parameters have been referred to in the text by names such as
LSRefreshTimer. The same naming convention is used for the configurable
protocol parameters. They are defined in appendix C.
The name of each architectural constant follows, together with its value
and a short description of its function.
LSRefreshTime
The maximum time between distinct originations of any particular
link state advertisement. For each link state advertisement that a
router originates, an interval timer should be set to this value.
Firing of this timer causes a new instance of the link state
advertisement to be originated. The value of LSRefreshTime is set
to 30 minutes.
MinLSInterval
The minimum time between distinct originations of any particular
link state advertisement. The value of MinLSInterval is set to 5
seconds.
MaxAge
The maximum age that a link state advertisement can attain. When an
advertisement's age reaches MaxAge, it is reflooded. It is then
removed from the database as soon as this flood is acknowledged,
i.e., as soon as it has been removed from all neighbor Link state
retransmission lists. Advertisements having age MaxAge are not used
in the routing table calculation. The value of MaxAge must be
greater than LSRefreshTime. The value of MaxAge is set to 1 hour.
CheckAge
When the age of a link state advertisement (that is contained in the
link state database) hits a multiple of CheckAge, the
advertisement's checksum is verified. An incorrect checksum at this
time indicates a serious error. The value of CheckAge is set to 5
minutes.
MaxAgeDiff
The maximum time dispersion that can occur, as a link state
advertisement is flooded throughout the AS. Most of this time is
accounted for by the link state advertisements sitting on router
output queues (and therefore not aging) during the flooding process.
The value of MaxAgeDiff is set to 15 minutes.
[Moy] [Page 164]RFC 1247 OSPF Version 2 July 1991
LSInfinity
The link state metric value indicating that the destination is
unreachable. It is defined to be the binary value of all ones. It
depends on the size of the metric field, which is 16 bits in router
links advertisements, and 24 bits in both summary and AS external
links advertisements.
DefaultDestination
The Destination ID that indicates the default route. This route is
used when no other matching routing table entry can be found. The
default destination can only be advertised in AS external link
advertisements and in type 3 summary link advertisements for stub
areas. Its value is the IP address 0.0.0.0.
[Moy] [Page 165]RFC 1247 OSPF Version 2 July 1991
C. Configurable Constants
The OSPF protocol has quite a few configurable parameters. These
parameters are listed below. They are grouped into general functional
categories (area parameters, interface parameters, etc.). Sample values
are given for some of the parameters.
Some parameter settings need to be consistent among groups of routers.
For example, all routers in an area must agree on that area's
parameters, and all routers attached to a network must agree on that
network's IP network number and mask.
Some parameters may be determined by router algorithms outside of this
specification (e.g., the address of a host connected to the router via a
SLIP line). From OSPF's point of view, these items are still
configurable.
C.1 Global parameters
In general, a separate copy of the OSPF protocol is run for each area.
Because of this, most configuration parameters are defined on a per-area
basis. The few global configuration parameters are listed below.
Router ID
This is a 32-bit number that uniquely identifies the router in the
Autonomous System. One algorithm for Router ID assignment is to
choose the largest or smallest IP address assigned to the router.
If a router's OSPF Router ID is changed, the router's OSPF software
should be restarted before the new Router ID takes effect.
TOS capability
This item indicates whether the router will calculate separate
routes based on TOS. For more information, see Sections 4.5 and
16.9.
C.2 Area parameters
All routers belonging to an area must agree on that area's
configuration. Disagreements between two routers will lead to an
inability for adjacencies to form between them, with a resulting
hindrance to the flow of routing protocol traffic. The following items
must be configured for an area:
[Moy] [Page 166]RFC 1247 OSPF Version 2 July 1991
Area ID
This is a 32-bit number that identifies the area. The Area ID of 0
is reserved for the backbone. If the area represents a subnetted
network, the IP network number of the subnetted network may be used
for the area ID.
List of address ranges
An OSPF area is defined as a list of [IP address, mask] pairs. Each
pair describes a range of IP addresses. Networks and hosts are
assigned to an area depending on whether their addresses fall into
one of the area's defining address ranges. Routers are viewed as
belonging to multiple areas, depending on their attached networks'
area membership. Routing information is condensed at area
boundaries. External to the area, a single route is advertised for
each address range.
As an example, suppose an IP subnetted network is to be its own OSPF
area. The area would be configured as a single address range, whose
IP address is the address of the subnetted network, and whose mask
is the natural class A, B, or C internet mask. A single route would
be advertised external to the area, describing the entire subnetted
network.
Authentication type
Each area can be configured for a separate type of
authentication. See Appendix E for a discussion of the
defined authentication types.
External routing capability
Whether AS external advertisements will be flooded into/throughout
the area. If AS external advertisements are excluded from the area,
the area is called a "stub". Internal to stub areas, routing to
external destinations will be based solely on a default summary
route. The backbone cannot be configured as a stub area. Also,
virtual links cannot be configured through stub areas. For more
information, see Section 3.6.
StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost indicates
the cost of the default summary link that the router should
advertise into the area. There can be a separate cost configured
for each IP TOS. See Section 12.4.3 for more information.
[Moy] [Page 167]RFC 1247 OSPF Version 2 July 1991
C.3 Router interface parameters
Some of the configurable router interface parameters (such as IP
interface address and subnet mask) actually imply properties of the
attached networks, and therefore must be consistent across all the
routers attached to that network. The parameters that must be
configured for a router interface are:
IP interface address
The IP protocol address for this interface. This uniquely
identifies the router over the entire internet. An IP address is
not required on serial lines. Such a serial line is called
"unnumbered".
IP interface mask
This denotes the portion of the IP interface address that
identifies the attached network. This is often referred to
as the subnet mask.
Interface output cost(s)
The cost of sending a packet on the interface, expressed in the link
state metric. This is advertised as the link cost for this
interface in the router's router links advertisement. There may be
a separate cost for each IP Type of Service. The interface output
cost(s) must always be greater than 0.
RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this interface. Also
used when retransmitting Database Description and Link State Request
Packets. This should be well over the expected round-trip delay
between any two routers on the attached network. The setting of
this value should be conservative or needless retransmissions will
result. It will need to be larger on low speed serial lines and
virtual links. Sample value for a local area network: 5 seconds.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface. Link state advertisements
contained in the update packet must have their age incremented by
this amount before transmission. This value should take into
account the transmission and propagation delays for the interface.
It must be greater than 0. Sample value for a local area network: 1
second.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
[Moy] [Page 168]RFC 1247 OSPF Version 2 July 1991
both attempt to become Designated Router, the one with the highest
Router Priority takes precedence. If there is still a tie, the
router with the highest Router ID takes precedence. A router whose
Router Priority is set to 0 is ineligible to become Designated
Router on the attached network. Router Priority is only configured
for interfaces to multi-access networks.
HelloInterval
The length of time, in seconds, between the Hello packets that the
router sends on the interface. This value is advertised in the
router's Hello packets. It must be the same for all routers
attached to a common network. The smaller the hello interval, the
faster topological changes will be detected, but more routing
traffic will ensue. Sample value for a X.25 PDN network: 30
seconds. Sample value for a local area network: 10 seconds.
RouterDeadInterval
The number of seconds that a router's Hellos have not been seen
before its neighbors declare the router down. This is also
advertised in the router's Hello Packets in the DeadInt field. This
should be some multiple of the HelloInterval (say 4). This value
again must be the same for all routers attached to a common network.
Authentication key
This configured data allows the authentication procedure to generate
and/or verify the authentication field in the OSPF header. For
example, if the authentication type indicates simple password, the
authentication key would be a 64-bit password. This key would be
inserted directly into the OSPF header when originating routing
protocol packets. There could be a separate password for each
network.
C.4 Virtual link parameters
Virtual links are used to restore/increase connectivity of the backbone.
Virtual links may be configured between any pair of area border routers
having interfaces to a common (non-backbone) area. The virtual link
appears as an unnumbered point-to-point link in the graph for the
backbone. The virtual link must be configured in both of the area
border routers.
A virtual link appears in router links advertisements (for the backbone)
as if it were a separate router interface to the backbone. As such, it
has all of the parameters associated with a router interface (see
Section C.3). Although a virtual link acts like an unnumbered point-
to-point link, it does have an associated IP interface address. This
address is used as the IP source in protocol packets it sends along the
[Moy] [Page 169]RFC 1247 OSPF Version 2 July 1991
virtual link, and is set dynamically during the routing table build
process. Interface output cost is also set dynamically on virtual links
to be the cost of the intra-area path between the two routers. The
parameter RxmtInterval must be configured, and should be well over the
expected round-trip delay between the two routers. This may be hard to
estimate for a virtual link. It is better to err on the side of making
it too large. Router Priority is not used on virtual links.
A virtual link is defined by the following two configurable parameters:
the Router ID of the virtual link's other endpoint, and the (non-
backbone) area through which the virtual link runs (referred to as the
virtual link's transit area). Virtual links cannot be configured
through stub areas.
C.5 Non-broadcast, multi-access network parameters
OSPF treats a non-broadcast, multi-access network much like it treats a
broadcast network. Since there many be many routers attached to the
network, a Designated Router is selected for the network. This
Designated Router then originates a networks links advertisement, which
lists all routers attached to the non-broadcast network.
However, due to the lack of broadcast capabilities, it is necessary to
use configuration parameters in the Designated Router selection. These
parameters need only be configured in those routers that are themselves
eligible to become Designated Router (i.e., those router's whose DR
Priority for the network is non-zero):
List of all other attached routers
The list of all other routers attached to the non-broadcast network.
Each router is listed by its IP interface address on the network.
Also, for each router listed, that router's eligibility to become
Designated Router must be defined. When an interface to a non-
broadcast network comes up, the router sends Hello packets only to
those neighbors eligible to become Designated Router, until the
identity of the Designated Router is discovered.
PollInterval
If a neighboring router has become inactive (hellos have not been
seen for RouterDeadInterval seconds), it may still be necessary to
send Hellos to the dead neighbor. These Hellos will be sent at the
reduced rate PollInterval, which should be much larger than
HelloInterval. Sample value for a PDN X.25 network: 2 minutes.
[Moy] [Page 170]RFC 1247 OSPF Version 2 July 1991
C.6 Host route parameters
Host routes are advertised in network links advertisements as stub
networks with mask 0xffffffff. They indicate either router interfaces
to point-to-point networks, looped router interfaces, or IP hosts that
are directly connected to the router (e.g., via a SLIP line). For each
host directly connected to the router, the following items must be
configured:
Host IP address
The IP address of the host.
Cost of link to host
The cost of sending a packet to the host, in terms of the link state
metric. There may be multiple costs configured, one for each IP
TOS. However, since the host probably has only a single connection
to the internet, the actual configured cost(s) in many cases is
unimportant (i.e., will have no effect on routing).
[Moy] [Page 171]RFC 1247 OSPF Version 2 July 1991
D. Required Statistics
An OSPF implementation must provide a minimum set of statistics
indicating the operational state of the protocol. These statistics must
be accessible to the user; this will probably be accomplished through
some sort of network management interface.
It is hoped that these statistics will aid in the debugging of the
implementation, and in the analysis of the protocol's performance.
The statistics can be broken into two broad categories. The first
consists of what we will call logging messages. These are messages
produced in real time, with generally a single message produced as the
result of a single protocol event. Such messages are also commonly
referred to as traps.
The second category will be referred to as cumulative statistics. These
are counters whose value have collected over time, such as the count of
link state retransmissions over the last hour. Also falling into this
category are dumps of the various routing data structures.
D.1 Logging messages
A logging message should be produced on every significant protocol
event. The major events are listed below. Most of these events
indicate a topological change in the routing domain. However, some
number of logging messages can be expected even when the routing domain
remains intact for long periods of time. For example, link state
originations will still happen due to the link state refresh timer
firing.
Any of the messages that refer to link state advertisements should print
the area associated with the advertisement. There is no area associated
with AS external link advertisements.
The following list of logging messages indicate topological changes in
the routing domain:
T1 The state of a router interface changes. Interface state changes
are documented in Section 9.3. In general, they will cause new link
state advertisements to be originated. The logging message produced
should include the interface's IP address (or other name), interface
type (virtual link, etc.) and old and new state values (as
documented in Section 9.1).
[Moy] [Page 172]RFC 1247 OSPF Version 2 July 1991
T2 The state of a neighbor changes. Neighbor state changes are
documented in Section 10.3. The logging message produced should
include the neighbor IP address, and old and new state values.
T3 The (Backup) Designated Router has changed on one of the attached
networks. See Section 9.4. The logging message produced should
include the network IP address, and the old and new (Backup)
Designated Routers.
T4 The router is originating a new instance of a link state
advertisement. The logging message produced should indicate the LS
type, Link State ID and Advertising Router associated with the
advertisement (see Section 12.4).
T5 The router has received a new instance of a link state
advertisement. The router receives these in Link State Update
packets. This will cause recalculation of the routing table. The
logging message produced should indicate the advertisement's LS
type, Link State ID and Advertising Router. The message should also
include the neighbor from whom the advertisement was received.
T6 An entry in the routing table has changed (see Section 11). The
logging message produced should indicate the Destination type,
Destination ID, and the old and new paths to the destination.
The following logging messages may indicate that there is a network
configuration error:
C1 A received OSPF packet is rejected due to errors in its IP/OSPF
header. The reasons for rejection are documented in Section 8.2.
They include OSPF checksum failure, authentication failure, and
inability to match the source with an active OSPF neighbor. The
logging message produced should include the IP source and
destination addresses, the router ID in the OSPF header, and the
reason for the rejection.
C2 An incoming Hello packet is rejected due to mismatches between the
Hello's parameters and those configured for the receiving interface
(see Section 10.5). This indicates a configuration problem on the
attached network. The logging message should include the Hello's
source, the receiving interface, and the non-matching parameters.
C3 An incoming Database Description packet, Link State Request Packet,
Link State Acknowledgment Packet or Link State Update packet is
rejected due to the source neighbor being in the wrong state (see
Sections 10.6, 10.7, 13.7 , and 13 respectively). This can be
[Moy] [Page 173]RFC 1247 OSPF Version 2 July 1991
normal when the identity of the network's Designated Router changes,
causing momentary disagreements over the validity of adjacencies.
The logging message should include the source neighbor, its state,
and the packet's type.
C4 A Database Description packet has been retransmitted. This may mean
that the value of RxmtInterval that has been configured for the
associated interface is too small. The logging message should
include the neighbor to whom the packet is being sent.
The following messages can be caused by packet transmission errors, or
software errors in an OSPF implementation:
E1 The checksum in a received link state advertisement is incorrect.
The advertisement is discarded (see Section 13). The logging
message should include the advertisement's LS type, Link State ID
and Advertising Router (which may be incorrect). The message should
also include the neighbor from whom the advertisement was received.
E2 During the aging process, it is discovered that one of the link
state advertisements in the database has an incorrect checksum.
This indicates memory corruption or a software error in the router
itself. The router should be dumped and restarted.
The following messages are an indication that a router has restarted,
losing track of its previous LS sequence number. Should these messages
continue, it may indicate the presence of duplicate Router IDs:
R1 Two link state advertisements have been seen, whose LS type, Link
State ID, Advertising Router and LS sequence number are the same,
yet with differing LS checksums. These are considered to be
different instances of the same advertisement. The instance with
the larger checksum is accepted as more recent (see Section 12.1.7,
13.1). The logging message should include the LS type, Link State
ID, Advertising Router, LS sequence number and the two differing
checksums.
R2 Two link state advertisements have been seen, whose LS type, Link
State ID, Advertising Router, LS sequence number and LS checksum are
the same, yet can be distinguished by their LS age fields. This
means that one of the advertisement's LS age is MaxAge, or the two
LS age fields differ by more than MaxAgeDiff. The logging message
should include the LS type, Link State ID, Advertising Router, LS
sequence number and the two differing ages.
[Moy] [Page 174]RFC 1247 OSPF Version 2 July 1991
R3 The router has received an instance of one of its self-originated
advertisements, that is considered to be more recent. This forces
the router to originate a new advertisement (see Section 13.4). The
logging message should include the advertisement's LS type, Link
State ID, and Advertising Router along with the neighbor from whom
the advertisement was received.
R4 An acknowledgment has been received for an instance of an
advertisement that is not currently contained in the router's
database (see Section 13.7). The logging message should detail the
instance being acknowledged and the database copy (if any), along
with the neighbor from whom the acknowledgment was received.
R5 An advertisement has been received through the flooding procedure
that is LESS recent the the router's current database copy (see
Section 13). The logging message should include the received
advertisement's LS type, Link State ID, Advertising Router, LS
sequence number, LS age and LS checksum. Also, the message should
display the neighbor from whom the advertisement was received.
The following messages are indication of normal, yet infrequent protocol
events. These messages will help in the interpretation of some of the
above messages:
N1 The Link state refresh timer has fired for one of the router's
self-originated advertisements (see Section 12.4). A new instance
of the advertisement must be originated. The message should include
the advertisement's LS type, Link State ID and Advertising Router.
N2 One of the advertisements in the router's link state database has
aged to MaxAge (see Section 14). At this point, the advertisement
is no longer included in the routing table calculation, and is
reflooded. The message should list the advertisement's LS type,
Link State ID and Advertising Router.
N3 An advertisement of age MaxAge has been flushed from the router's
database. This occurs after the advertisement has been acknowledged
by all adjacent neighbors. The message should list the
advertisement's LS type, Link State ID and Advertising Router.
D.2 Cumulative statistics
These statistics display collections of the routing data structures.
They should be able to be obtained interactively, through some kind of
network management facility.
[Moy] [Page 175]RFC 1247 OSPF Version 2 July 1991
All the following statistics displays, with the exception of the area
list, routing table and the AS external links, are specific to a single
area. As noted in Section 4, most OSPF protocol mechanisms work on each
area separately.
The following statistics displays should be available:
(1) A list of all the areas attached to the router, along with the
authentication type to use for the area, the number of router
interfaces attaching to the area, and the total number of nets and
routers belonging to the area.
For example, consider the router RT3 pictured in Figure 15. It has
interfaces to two separate areas, Area 1 and the backbone (Area 0).
Table 20 then indicates that the backbone is using a simple password
for authentication, and that Area 1 is not using any authentication.
The number of nets includes IP networks, subnets, and hosts (this is
the reason for 2 backbone nets -- they are the host routes
corresponding to the serial line between backbone routers RT6 and
RT10).
Area ID # ifcs AuType # nets # routers
______________________________________________
0 1 1 2 7
1 2 0 4 4
Table 20: Sample OSPF area display.
(2) A list of all the router's interfaces to an area, along with their
addresses, output cost, current state, the (Backup) Designated
Router for the attached network, and the number of neighbors
currently associated with the interface. Some number of these
neighbors will have become adjacent, the number of these is noted in
the display also.
Again consider router RT3 in Figure 15. Table 21 below indicates
that RT4 has been selected as Designated Router for network N3, and
router RT1 has been selected as Backup. Adjacencies have been
established to both of these routers. There are no routers besides
RT3 attached to network N4, so it becomes DR, yet still advertises
the network as a stub in its router links advertisements.
[Moy] [Page 176]RFC 1247 OSPF Version 2 July 1991
Ifc IP address state cost DR Backup # nbrs # adjs
__________________________________________________________________________
192.1.1.3 DR other 1 192.1.1.4 192.1.1.1 3 2
192.1.4.3 DR 2 192.1.4.3 none 0 0
Table 21: Sample OSPF interface display.
(3) The list of neighbors associated with a particular interface. Each
neighbor's IP address, router ID, state, and the length of the three
link state advertisement queues (see Section 10) to the neighbor is
displayed.
Suppose router RT4 is the Designated Router for network N3, and
router RT1 is the Backup Designated router. Suppose also that the
adjacency between router RT3 and RT1 has not yet fully formed. The
display of router RT3's neighbors (associated with its interface to
network N3) may then look like Table 22. The display indicates that
RT3 and RT1 are still in the database exchange procedure, Router RT3
has more Database Description packets to send to RT1, and RT1 has at
least one link state advertisement that RT3 doesn't. Also, there is
a single link state advertisement that has been flooded, but not
acknowledged, to each neighbor that participates in the flooding
procedure (state >= Exchng). (In the following examples we assume
that a router's Router ID is assigned to be its smallest IP
interface address).
Nbr IP address Router ID state LS rxmt len DB summ len LS req len
____________________________________________________________________________
192.1.1.1 192.1.1.1 Exchng 1 10 1
192.1.1.2 192.1.1.2 2-Way 0 0 0
192.1.1.4 192.1.1.4 Full 1 0 0
Table 22: Sample OSPF neighbor display.
(4) A list of the area's link state database. This is the same in all
of the routers attached to the area. It is composed of that area's
router links, network links, and summary links advertisements.
Also, the AS external link advertisements are a part of all the
areas' databases.
[Moy] [Page 177]RFC 1247 OSPF Version 2 July 1991
The link state database for Area 1 in Figure 15 might look like
Table 23 (compare this with Figure 7). Assume the the Designated
Router for network N3 is router RT4, as above. Both routers RT3 and
RT4 are originating summary link advertisements into Area 1, since
they are area border routers. Routers RT5 and RT7 are AS external
routers. Their location must be described in summary links
advertisements. Also, their AS external link advertisements are
flooded throughout the entire AS.
Router RT3 can locate its self-originated advertisements by looking
for its own router ID (192.1.1.3) in advertisements' Advertising
Router fields.
The LS sequence number, LS age, and LS checksum fields indicate the
advertisement's instance. Their values are stored in the
advertisement's link state header; we have not bothered to make up
values for the example.
LS type Link State ID Advertising Router LS seq no LS age LS checksum
_______________________________________________________________________________
1 192.1.1.1 192.1.1.1 * * *
1 192.1.1.2 192.1.1.2 * * *
1 192.1.1.3 192.1.1.3 * * *
1 192.1.1.4 192.1.1.4 * * *
_______________________________________________________________________________
2 192.1.1.4 192.1.1.4 * * *
_______________________________________________________________________________
3 Ia,Ib 192.1.1.3 * * *
3 N6 192.1.1.3 * * *
3 N7 192.1.1.3 * * *
3 N8 192.1.1.3 * * *
3 N9-N11,H1 192.1.1.3 * * *
3 Ia,Ib 192.1.1.4 * * *
3 N6 192.1.1.4 * * *
3 N7 192.1.1.4 * * *
3 N8 192.1.1.4 * * *
3 N9-N11,H1 192.1.1.4 * * *
4 RT5 192.1.1.3 * * *
4 RT7 192.1.1.3 * * *
4 RT5 192.1.1.4 * * *
4 RT7 192.1.1.4 * * *
_______________________________________________________________________________
4 N12 RT5's ID * * *
4 N13 RT5's ID * * *
4 N14 RT5's ID * * *
4 N12 RT7's ID * * *
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LS type Link State ID Advertising Router LS seq no LS age LS checksum
_______________________________________________________________________________
4 N15 RT7's ID * * *
Table 23: Sample OSPF link state database display.
(5) The contents of any particular link state advertisement. For
example, a listing of the router links advertisement for Area 1,
with LS type = 1 and Link State ID = 192.1.1.3 is shown in Section
12.4.1.
(6) A listing of the entire routing table. Several examples are shown
in Section 11. The routing table is calculated from the combined
databases of each attached area (see Section 16). It may be
desirable to sort the routing table by Type of Service, or by
destination, or a combination of the two.
[Moy] [Page 179]RFC 1247 OSPF Version 2 July 1991
E. Authentication
All OSPF protocol exchanges are authenticated. The OSPF packet header
(see Section A.3.1) includes an authentication type field, and 64-bits
of data for use by the appropriate authentication scheme (determined by
the type field).
The authentication type is configurable on a per-area basis. Additional
authentication data is configurable on a per-interface basis. For
example, if an area uses a simple password scheme for authentication, a
separate password may be configured for each network contained in the
area.
Authentication types 0 and 1 are defined by this specification. All
other authentication types are reserved for definition by the IANA
(iana@ISI.EDU). The current list of authentication types is described
below in Table 24.
AuType Description
_______________________________________________________________
0 No authentication
1 Simple password
All others Reserved for assignment by the IANA (iana@ISI.EDU)
Table 24: OSPF authentication types.
E.1 Autype 0 -- No authentication
Use of this authentication type means that routing exchanges in the area
are not authenticated. The 64-bit field in the OSPF header can contain
anything; it is not examined on packet reception.
E.2 Autype 1 -- Simple password
Using this authentication type, a 64-bit field is configured on a per-
network basis. All packets sent on a particular network must have this
configured value in their OSPF header 64-bit authentication field. This
essentially serves as a "clear" 64-bit password.
This guards against routers inadvertently coming up in the area. They
must first be configured with their attached networks' passwords before
they can join the routing domain.
[Moy] [Page 180]RFC 1247 OSPF Version 2 July 1991
F. Version 1 differences
This section documents the changes between OSPF version 1 and OSPF
version 2. The impetus for these changes derives from comments received
on RFC 1131 and recent field experience with the OSPF protocol.
Unfortunately, the changes are not backward-compatible. For that
reason, OSPF version 1 will not interoperate with OSPF version 2.
However, the changes are small in scope and should not greatly affect
any existing implementations. In addition, some of the proposed changes
should enable future protocol additions to be made in a backward-
compatible manner (see Section F.4).
F.1 Protocol Enhancements
The following enhancements were made to the OSPF protocol.
F.1.1 Stub area support
In many Autonomous Systems, the majority of the OSPF link state database
consists of AS external advertisements. In these Autonomous Systems,
some OSPF areas may be organized in such a way that external
advertisements can be safely ignored, enabling a reduction of the area's
database size. This applies to OSPF areas where there is only a single
exit/entry that is used by all externally addressed packets, or to cases
where some sub-optimality of external routing is acceptable.
Therefore, an OSPF area configuration option has been added (see
Sections 3.6 and C.2) allowing the import of external advertisements to
be disabled for an area. When this option is enabled, no AS external
advertisements will be flooded into the area (Sections 13, 13.3 and
10.3). Instead, within the area all data traffic to external
destinations will follow a (per-area) default route. These areas are
called "stub" areas.
To implement this, all area border routers attached to stub areas will
originate a default summary link advertisement for the area (Section
12.4.3). This will direct all internal routers to an area border router
when forwarding externally addressed packets. In addition, to ensure
that stub areas are configured consistently, an Options field has been
added to OSPF Hello packets (Sections A.2 and A.3.2). A bit is reset in
the Options field indicating that the attached area is a stub area
(Section 9.5). A router will not accept a neighbor's hellos unless they
both agree on the area's ability to process AS external advertisements
(Section 10.5). In this way, a system administrator will be able to
discover incorrectly configured routers, and data traffic will be routed
around them (in order to avoid potential looping situations) until their
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configuration can be repaired.
F.1.2 Optional TOS support
In OSPF there is conceptually a separate routing table for each TOS; the
calculations detailed in steps 1-5 of Section 16 must be done separately
for each TOS. (Note however that link and summary costs need not be
specified separately for each TOS; costs for unspecified TOS values
default to the cost of TOS 0).
In version 1 of the OSPF specification, all OSPF routers were required
to route based on TOS. However, producing a separate routing table for
each TOS may prove costly, both in terms of memory and processor
resources. For this reason, version 2 allows the system administrator
to configure routers to calculate/use only a single routing table (the
TOS 0 table). When this is done, some traffic may take non-optimal
routes. But all packets will still be delivered, and routing will
remain loop free (see Section 2.4).
In order to avoid routing loops, a router (router X) using a single
table must communicate this information to its peers. This is done by
resetting the new TOS-capable bit in the router X's router links
advertisement (Section 12.4.1). Then, when its peers perform the
Dijkstra calculation (Section 16.1) for non-zero TOS values, they will
omit router X from the calculation. In effect, an attempt will be made
to bypass router X when forwarding non-zero TOS traffic. Summary link
and AS external link advertisements can also indicate their non-
availability for non-zero TOS traffic (Sections 12.4.3 and 12.4.4).
The result may be that no route can be found for some non-zero value of
TOS. When this happens, the packet is routed along the TOS 0 route
instead (Section 11.1).
It is still mandatory for all OSPF implementations to be able to
construct separate routing tables for each TOS value, if desired by the
system administrator.
F.1.3 Preventing external extra-hops
In some cases, version 1 of the OSPF specification will introduce
extra-hops when calculating routes to external destinations. This is
because it is implicit in the format of AS external advertisements that
packets should be forwarded through the advertising router. However,
consider the situation where multiple OSPF routers share a LAN with an
external router (call it router Y) , and only one OSPF router (call it
router X) exchanges routing information with Y. The OSPF routers on the
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LAN other than X will forward packets destined for Y and beyond through
X, generating an extra hop (see Section 2.2).
To fix this, a new field has been added to AS external advertisements.
This field (called the forwarding address) will indicate the router
address to which packets should be forwarded (Section 12.4.4). In the
above example, router X will put Y's IP address into this field. If the
field is 0, packets are (as before) forwarded to the originator of the
advertisement. A different forwarding address can be specified for each
TOS value.
Whenever possible, this new field should be set to 0. This is because
setting it to an actual router address incurs additional cost during the
routing table build process (Section 16.4).
Besides preventing extra-hops, there are two other applications for this
field. The first is for use by "route servers". Using the forwarding
address, a router in the middle of the Autonomous System can gather
external routing information and originate AS external advertisements
that specify the correct exit route to use for each external destination
(Section 2.2).
The other application possibly enables the reduction of the number of AS
external advertisements that need be imported. Suppose in the example
at the beginning of this section that there are two routers (X and Z)
exchanging EGP information with the non-OSPF router Y. It is then
likely that both X and Z will originate the same set of external routes.
Two AS external advertisements that specify the same (non-zero)
forwarding address, destination and cost are obviously functionally
equivalent, regardless of their originators (advertising routers). The
OSPF specification dictates that the advertisement originated by the
router with the largest Router ID will always be used. This allows the
other router to flush its equivalent advertisement (Section 12.4.4).
F.2 Corrected problems
The following problems in OSPF version 1 have been corrected in version
2.
F.2.1 LS sequence number space changes
The LS sequence number space has been changed from version 1's lollipop
shape to a linear sequence space (Section 12.1.6). Sequence numbers
will now be compared as signed 32-bit integers. Link state
advertisements having larger sequence numbers will be considered more
recent. The sequence number space will still begin at (-N+1) (where N =
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2**31). The value of -N remains reserved. The LS sequence number of
successive instances of an advertisement will continue to be incremented
until it reaches the maximum possible value: N-1. At this point, when a
new instance of the advertisement must be originated (due either to
topological change of the expiration of the LS refresh timer) the
current instance must first be "prematurely aged".
There will be a new section discussing premature aging (Section 14.1).
This is a method for flushing a link state advertisement from the
routing domain: the advertisement's age is set to MaxAge and
advertisement is reflooded just as if it were a newly received
advertisement. As soon as the new flooding is acknowledged by all of
the router's adjacent neighbors, the advertisement is flushed from the
database.
Premature aging can also be used when, for example, a previously
advertised external route is no longer reachable. In this circumstance,
premature aging is preferable to the alternative, which is to originate
a new advertisement for the destination specifying a metric of
LSInfinity.
A router may only prematurely age its own (self-originated) link state
advertisements. These are the link state advertisements having the
router's own OSPF router ID in the Advertising Router field.
F.2.2 Flooding of unexpected MaxAge advertisements
Version 1 of the OSPF omitted the handling of a special case in the
flooding procedure: the reception of a MaxAge advertisement that has no
database instance. A paragraph has been added to Section 13 to deal
with this occurrence. Without this paragraph, retransmissions of MaxAge
advertisements could possibly delay their being flushed from the routing
domain.
F.2.3 Virtual links and address ranges
When summarizing information into a virtual link's transit area, version
2 of the OSPF specification prohibits the collapsing of multiple
backbone IP networks/subnets into a single summary link. This
restriction has been added to deal with certain anomalous OSPF area
configurations. See Sections 15 and 12.4.3 for more information.
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RFC 1247 OSPF Version 2 July 1991
F.2.4 Routing table lookup explained
When forwarding an IP data packet, a router looks up the packet's IP
destination in the routing table. This determines the packet's next
hop. A new section (Section 11.1) has been added describing the routing
table lookup (instead of just specifying a "best match"). This section
clarifies OSPF's four level routing hierarchy (i.e., intra-area, inter-
area, external type 1 and external type 2 routes). It also specifies
the effect of TOS on routing.
F.2.5 Sending Link State Request packets
OSPF Version 2 eases the restrictions on the sending of Link State
Request packets. Link State Request packets can now be sent to a
neighboring router before a complete set of Database Description packets
have been exchanged. This enables a more efficient use of a router's
memory resources; an OSPF version 2 implementation may limit the size of
the neighbor Link state request lists. See Sections 10.9, 10.7 and 10.3
for more details.
F.2.6 Changes to the Database description process
The specification has been modified to ensure that, when two routers are
synchronizing their databases during the Database Description process,
none of the component link state advertisements can have their sequence
numbers decrease. A link state advertisement's sequence number
decreases when it is flushed from the routing domain via premature-
aging, and then reoriginated with the smallest sequence number
0x80000001 (see Section 14.1). So the specification now dictates that
an advertisement cannot be flushed from a router's database until both
a) it no longer appears on any neighbor Link State Retransmission lists
and b) none of the router's neighbors are in states Exchange or Loading.
See Sections 13 (step 4c) and 14.1 for more details.
In addition, a new step has been added to the flooding procedure
(Section 13) in order to make the Database Description process more
robust. This step detects when a neighbor lists one instance of an
advertisement in its Database Description packets, but responds to Link
State Request packets by sending another (earlier) instance. This
behavior now causes the event BadLSReq to be generated, which restarts
the Database Description process with the neighbor. In OSPF version 1,
the neighbor event BadLSReq erroneously did not restart the Database
Description process.
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F.2.7 Receiving OSPF Hello packets
The section detailing the receive processing of OSPF Hello packets
(Section 10.5) has been modified to include the generation of the
neighbor Backup Seen event. In addition, the section detailing the
Designated Router election algorithm (Section 9.4) has been modified to
include the algorithm's initial state.
F.2.8 Network mask defined for default route
The network mask for the default route, when it appears as the
destination in either an AS external link advertisement or in a summary
link advertisement, has been set to 0.0.0.0. See Sections A.4.4 and
A.4.5 for more details.
F.2.9 Rate limit imposed on flooding
When an advertisement is installed in the link state database, it is
timestamped. The flooding procedure is then not allowed to install a
new instance of the advertisement until MinLSInterval seconds have
elapsed. This enforces a rate limit on the flooding procedure; a new
instance can be flooded only once every MinLSInterval seconds. This
guards against routers that disregard the limit on self-originated
advertisements (already present in OSPF version 1) of one origination
every MinLSInterval seconds. For more information, see Section 13.
F.3 Packet format changes
The following changes have been made to the format of OSPF packets and
link state advertisements. Some of these changes were required to
support the added functionality listed above. Other changes were made
to further simplify the parsing of OSPF packets.
F.3.1 Adding a Capability bitfield
To support the new "stub area" and "optional TOS" features, a bitfield
listing protocol capabilities has been added to the Hello packet,
Database Description packet and all link state advertisements. When
used in Hello packets, this allows a router to reject a neighbor because
of a capability mismatch. Alternatively, when capabilities are
exchanged in Database Description packets a router can choose not to
forward certain link state advertisements to a neighbor because of its
reduced functionality. Lastly, listing capabilities in link state
advertisements allows routers to route traffic around reduced
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functionality router, by excluding them from parts of the routing table
calculation. See Section A.2 for more details.
F.3.2 Packet simplification
To simplify the format of Database Description packets and Link State
Acknowledgment packets, their description of link state advertisements
has been modified. Each advertisement is now be described by its 20-
byte link state header (see Section A.4). This does not consume any
additional space in the packets. The one additional piece of
information that will be present is the LS length. However, this field
need not be used when processing the Database Description and Link State
Acknowledgment packets.
F.3.3 Adding forwarding addresses to AS external advertisements
As discussed in Section F.1.3, a forwarding address field has been added
to the AS external advertisement.
F.3.4 Labelling of virtual links
Virtual links will be labelled as such in router links advertisements.
This separates virtual links from unnumbered point-to-point links,
allowing all backbone routers to discover whether any virtual links are
in use. See Section 12.4.1 for more details.
F.3.5 TOS costs ordered
When a link state advertisement specifies a separate cost depending on
TOS, these costs must be ordered by increasing TOS value. For example,
the cost for TOS 16 must always follow the cost for TOS 8.
F.3.6 OSPF's TOS encoding redefined
The way that OSPF encodes TOS in its link state advertisements has been
redefined in version 2. OSPF's encoding of the Delay (D), Throughput (T)
and Reliability (R) TOS flags defined by [RFC 791] is described in
Section 12.3.
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F.4 Backward-compatibility provisions
Additional functionality will probably be added to OSPF in the future.
One example of this is a multicast routing capability, which is
currently under development. In order to be able to add such features
in a backward-compatible manner, the following provisions have been made
in the OSPF specification.
New capabilities will probably involve the introduction of new link
state advertisements. If a router receives a link state advertisement
of unknown type during the flooding procedure, the advertisement is
simply ignored (Section 13. The router should not attempt to further
flood the advertisement, nor acknowledge it. The advertisement should
not be installed into the link state database. If the router receives
an advertisement of unknown type during the Database Description
process, this is an error (see Sections 10.6 and 10.3). The Database
Description process is then restarted.
There is also an Options field in both the Hello packets, Database
Description packets and the link state advertisement headers.
Unrecognized capabilities found in these places should be ignored, and
should not affect the normal processing of protocol packets/link state
advertisements (see Sections 10.5 and 10.6). Routers will originate
their Hello packets, Database Description packets and link state
advertisements with unrecognized capabilities set to 0 (see Sections
9.5, 10.8 and 12.1.2).
[Moy] [Page 188]
RFC 1247 OSPF Version 2 July 1991
Security Considerations
All OSPF protocol exchanges are authenticated. This is accomplished
through authentication fields contained in the OSPF packet header. For
more information, see Sections 8.1, 8.2, and Appendix E.
Author's Address
John Moy
Proteon, Inc.
2 Technology Drive
Westborough, MA 01581
Phone: (508) 898-2800
EMail: jmoy@proteon.com
[Moy] [Page 189]
