Network Working Group K. Varadhan
Request for Comments: 1364 OARnet
September 1992
BGP OSPF Interaction
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 defines the various criteria to be used when designing
Autonomous System Border Routers (ASBR) that will run BGP with other
ASBRs external to the AS and OSPF as its IGP.
Table of Contents
1. Introduction ................................................. 2
2. Route Exchange ............................................... 2
2.1. Exporting OSPF routes into BGP ............................. 3
2.2. Importing BGP routes into OSPF ............................. 4
3. BGP Identifier and OSPF router ID ............................ 5
4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes ......... 5
4.1. Semantics of the characteristics bits ...................... 7
4.2. Configuration parameters for setting the OSPF tag .......... 8
4.3. Manually configured tags ................................... 9
4.4. Automatically generated tags ................................ 9
4.4.1. Routes with incomplete path information, pl = 0 ........... 9
4.4.2. Routes with incomplete path information, pl = 1 ........... 9
4.4.3. Routes with incomplete path information, pl >= 1 ..........10
4.4.4. Routes with complete path information, pl = 0 .............10
4.4.5. Routes with complete path information, pl = 1 .............11
4.4.6. Routes with complete path information, pl >= 1 ............11
4.5. Miscellaneous tag settings ..................................12
4.6. Summary of the TagType field setting ........................12
5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute ....12
6. Security Considerations .......................................13
7. Acknowledgements ..............................................13
8. Bibliography ..................................................14
9. Author's Address ..............................................14
Varadhan [Page 1]RFC 1364 BGP OSPF Interaction September 1992
1. Introduction
This document defines the various criteria to be used when designing
Autonomous System Border Routers (ASBR) that will run BGP [RFC1267]
with other ASBRs external to the AS, and OSPF [RFC1247] as its IGP.
This document defines how the following fields in OSPF and attributes
in BGP are to be set when interfacing between BGP and OSPF at an
ASBR:
OSPF cost and type vs. BGP INTER-AS METRIC
OSPF tag vs. BGP ORIGIN and AS_PATH
OSPF Forwarding Address vs. BGP NEXT_HOP
For a more general treatise on routing and route exchange problems,
please refer to [ROUTE-LEAKING] and [NEXT-HOP] by Philip Almquist.
This document uses the two terms "Autonomous System" and "Routing
Domain". The definitions for the two are below:
The term Autonomous System is the same as is used in the BGP-3 RFC
[RFC1267], given below:
"The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the
administration of an AS appears to other ASs to have a single
coherent interior routing plan and presents a consistent picture
of what networks are reachable through it. From the standpoint of
exterior routing, an AS can be viewed as monolithic: reachability
to networks directly connected to the AS must be equivalent from
all border gateways of the AS."
The term Routing Domain was first used in [ROUTE-LEAKING] and is
given below:
"A Routing Domain is a collection of routers which coordinate
their routing knowledge using a single (instance of) a routing
protocol."
2. Route Exchange
This section discusses the constraints that must be met to exchange
routes between an external BGP session with a peer from another AS
and internal OSPF routes.
BGP does not carry subnet information in routing updates. Therefore,
when referring to a subnetted network in the OSPF routing domain, we
consider the equivalent network route in the context of BGP.
Varadhan [Page 2]RFC 1364 BGP OSPF Interaction September 1992
Multiple subnet routes for a subnetted network in OSPF are collapsed
into one network route when exported into BGP.
2.1. Exporting OSPF routes into BGP
1. The administrator must be able to selectively export routes
into BGP via an appropriate filter mechanism.
This filter mechanism must support such control with the
granularity of a single network.
Additionally, the administrator must be able to filter based
on the OSPF tag and the various sub-fields of the OSPF tag.
The settings of the tag and the sub-fields are defined in
section 4 in more detail.
o By default, no routes must be exported from OSPF into
BGP. A single mechanism must permit all OSPF inter-area
and intra-area routes to be exported into BGP.
OSPF external routes of type 1 and type 2 must never be
exported into BGP unless they are explicitly configured.
2. When configured to export a network, the ASBR must advertise
a network route for a subnetted network, as long as at least
one subnet in the subnetted network is reachable via OSPF.
3. The network administrator must be able to statically
configure the BGP attribute INTER-AS METRIC to be used for
any network route.
o By default, the INTER_AS METRIC must default to 1.
Explanatory text: The OSPF cost and the BGP INTER-AS METRIC
are of different widths. The OSPF cost is a two level
metric. The BGP INTER-AS METRIC is only an optional non-
transitive attribute. Hence, a more complex BGP INTER-AS
METRIC-OSPF cost mapping scheme is not necessary.
4. When an ASBR is advertising an OSPF route to network Y to
external BGP neighbours and learns that the route has become
unreachable, the ASBR must immediately propogate this
information to the external BGP neighbours.
5. An implementation of BGP and OSPF on an ASBR must have a
mechanism to set up a minimum amount of time that must elapse
between the learning of a new route via OSPF and subsequent
advertisement of the route via BGP to the external
Varadhan [Page 3]RFC 1364 BGP OSPF Interaction September 1992
neighbours.
o The default value for this setting must be 0, indicating
that the route is to be advertised to the neighbour BGP
peers instantly.
Note that [RFC1267] mandates a mechanism to dampen the
inbound advertisements from adjacent neighbours.
2.2. Importing BGP routes into OSPF
1. BGP implementations should allow an AS to control
announcements of BGP-learned routes into OSPF.
Implementations should support such control with the
granularity of a single network. Implementations should also
support such control with the granularity of an autonomous
system, where the autonomous system may be either the
autonomous system that originated the route or the autonomous
system that advertised the route to the local system
(adjacent autonomous system).
o By default, no routes must be imported from BGP into
OSPF. Administrators must configure every route they
wish to import.
A mechanism may allow an administrator to configure an
ASBR to import all the BGP routes into the OSPF routing
domain.
2. The administrator must be able to configure the OSPF cost and
the OSPF metric type of every route imported into OSPF.
o The OSPF cost must default to 1; the OSPF metric type
must default to type 2.
3. Routes learned via IBGP must not be imported into OSPF.
4. The ASBR must never generate a default route into the OSPF
routing domain unless explicitly configured to do so.
A possible criterion for generating default into an IGP is to
allow the administrator to specify a set of (network route,
AS_PATH, default route cost, default route type) tuples. If
the ASBR learns of the network route for an element of the
set, with the corresponding AS_PATH, then it generates a
default route into the OSPF routing domain, with cost
"default route cost" and type, "default route type". The
lowest cost default route will then be injected into the OSPF
Varadhan [Page 4]RFC 1364 BGP OSPF Interaction September 1992
routing domain.
This is the recommended method for originating default routes
in the OSPF routing domain.
3. BGP Identifier and OSPF router ID
The BGP identifier must be the same as the OSPF router id at all
times that the router is up.
This characteristic is required for two reasons.
i. Consider the scenario in which 3 routers, RT1, RT2, and RT3,
belong to the same autonomous system.
+-----+
| RT3 |
+-----+
|
Autonomous System running OSPF
/ \
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
Both RT1 and RT2 have routes to an external network X and import it
into the OSPF routing domain. RT3 is advertising the route to
network X to other external BGP speakers. RT3 must use the OSPF
router ID to determine whether it is using RT1 or RT2 to forward
packets to network X and hence build the correct AS_PATH to advertise
to other external speakers.
More precisely, RT3 must use the AS_PATH of the route announced by
the ASBR, whose BGP Identifier is the same as the OSPF routerID
corresponding to its route for network X.
ii. It will be convenient for the network administrator looking at
an ASBR to correlate different BGP and OSPF routes based on
the identifier.
4. Setting OSPF tags, BGP ORIGIN and AS_PATH attributes
The OSPF external route tag is a "32-bit field attached to each
external route . . . It may be used to communicate information
between AS boundary routers; the precise nature of such information
is outside the scope of [the] specification." [RFC1247]
Varadhan [Page 5]RFC 1364 BGP OSPF Interaction September 1992
OSPF imports information from various routing protocols at all its
ASBRs. In some instances, it is possible to use protocols other than
EGP or BGP across autonomous systems. It is important, in BGP, to
differentiate between routes that are external to the OSPF routing
domain but must be considered internal to the AS, as opposed to
routes that are external to the AS.
Routes that are internal to the AS and that may or may not be
external to the OSPF routing domain will not come to the various BGP
speakers via IBGP. Therefore, ASBRs running BGP must have knowledge
of this class of routes so that they can advertise these routes to
the various external AS without waiting for IBGP updates about these
routes.
Additionally, in the specific instance of an AS intermixing routers
running EGP and BGP as external gateway routing protocols, using OSPF
as an IGP, the network administrator does not have to configure IBGP
on every ASBR running EGP and not running BGP, if this information
can be carried in the OSPF tag field.
We use the external route tag field in OSPF to intelligently set the
ORIGIN and AS_PATH attributes in BGP. Both the ORIGIN and AS_PATH
attributes are well-known, mandatory attributes in BGP. The exact
mechanism for setting the tags is defined below.
The tag is broken up into sub-fields shown below. The various sub-
fields specify the characteristics of the route imported into the
OSPF routing domain.
The high bit of the OSPF tag is known as the "Automatic" bit. When
this bit is set to 1, the following sub-fields apply:
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|a|c|p l| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
a is 1 bit called the Automatic bit, indicating that the
Completeness and PathLength bits have been generated
automatically by a router. The meaning of this characteristic
and its setting are defined below.
c is 1 bit of Completeness information. The meaning of this
characteristic and its settings are defined below.
pl are 2 bits of PathLength information. The meaning of this
characteristic and its setting are defined below.
Varadhan [Page 6]RFC 1364 BGP OSPF Interaction September 1992
ArbitraryTag (or "at")
is 12 bits of tag information, which defaults to 0 but can be
configured to anything else.
AutonomousSystem (or "as")
is 16 bits, indicating the AS number corresponding to the
route, 0 if the route is to be considered as part of the local
AS.
When the Automatic bit is set to 0, the following sub-fields apply:
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|a| LocalInfo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
a is 1 bit called the Automatic bit, set to 0.
LocalInfo (or "li")
is 31 bits of an arbitrary value, manually configured by the
network administrator.
The format of the tag for various values of the characteristics bits
is defined below.
4.1. Semantics of the characteristics bits
The Completeness and PathLength characteristics bits define the
characteristic of the route imported into OSPF from other ASBRs in
the autonomous system. This setting is then used to set the ORIGIN
and NEXT_HOP attributes when re-exporting these routes to an external
BGP speaker.
o The "a" bit or the Automatic characteristic bit is set when
the Completeness and PathLength characteristics bits are
automatically set by a border router.
For backward compatibility, the Automatic bit must default to
0 and the network administrator must have a mechanism to
enable automatic tag generation. Nothing must be inferred
about the characteristics of the OSPF route from the tag
bits, unless the tag has been automatically generated.
o The "c" bit of the Completeness characteristic bit is set
when the source of the incoming route is known precisely, for
instance, from an IGP within the local autonomous system or
EGP at one of the autonomous system's boundaries. It refers
Varadhan [Page 7]RFC 1364 BGP OSPF Interaction September 1992
to the status of the path information carried by the routing
protocol.
o The "pl" or the PathLength characteristic sub-field is set
depending on the length of the AS_PATH that the protocol
could have carried when importing the route into the OSPF
routing domain. The length bits will indicate whether the
AS_PATH attribute for the length is zero, one, or greater
than one.
Routes imported from an IGP will usually have an AS_PATH of
length of 0, routes imported from an EGP will have an AS_PATH
of length 1, BGP and routing protocols that support complete
path information, either as AS_PATHs or routing domain paths,
will indicate a path greater than 1.
The OSPF tag is not wide enough to carry path information
about routes that have an associated PathLength greater than
one. Path information about these routes will have to be
carried via IBGP. Such routes must not be exported from OSPF
into BGP.
For brevity in the following sections, the keywords O and P refer to
the BGP ORIGIN and AS_PATH attributes respectively. Likewise, we use
the abbreviations , "l" and "nh" for the local_AS and next_hop_AS
respectively in the following sections.
4.2. Configuration parameters for setting the OSPF tag
o There must be a mechanism to enable automatic generation of
the tag characteristic bits.
o Configuration of an ASBR running OSPF must include the
capability to associate a tag value, for the ArbitraryTag, or
LocalInfo sub-field of the OSPF tag, with each instance of a
routing protocol.
o Configuration of an ASBR running OSPF must include the
capability to associate an AS number with each instance of a
routing protocol.
Associating an AS number with an instance of an IGP is
equivalent to flagging those set of routes imported from the
IGP to be external routes outside the local autonomous
system.
Specifically, when the IGP is RIP [RFC1058], it should be
possible to associate a tag and/or an AS number with every
Varadhan [Page 8]RFC 1364 BGP OSPF Interaction September 1992
interface running RIP on the ASBR.
4.3. Manually configured tags
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| LocalInfo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This tag setting corresponds to the administrator manually setting
the tag bits. Nothing shall be inferred about the characteristics of
the route corresponding to this tag setting.
For backward compatibility with existing implementations of OSPF
currently deployed in the field, this must be the default setting for
importing routes into the OSPF routing domain. There must be a
mechanism to enable automatic tag generation for imported routes.
The OSPF tag to BGP attribute mappings for these routes must be
a=0, li=Arbitrary_Value => O=<INCOMPLETE>, P=<l>
4.4. Automatically generated tags
4.4.1. Routes with incomplete path information, pl = 0.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with incomplete
path information and cannot or may not carry the neighbour AS or
AS path as part of the routing information.
The OSPF tag to BGP attribute mappings for these routes must be
a=1,c=0,pl=00,as=0 => O=<EGP>, P=<l>
4.4.2 Routes with incomplete path information, pl = 1.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|1| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with incomplete
Varadhan [Page 9]RFC 1364 BGP OSPF Interaction September 1992
path information and carry the neighbour AS as part of the routing
information.
The OSPF tag to BGP attribute mappings for these routes must be
a=1,c=0,pl=01,as=nh => O=<EGP>, P=<l, nh>
This setting should be used for importing EGP routes into the OSPF
routing domain. This setting can also be used when importing BGP
routes whose origin=<EGP> and AS_PATH=<nh>; if the BGP learned
route has no other transitive attributes, then its propogation via
IBGP can be suppressed.
4.4.3. Routes with incomplete path information, pl >= 1.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|1|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with truncated
path information.
The OSPF tag to BGP attribute mappings for these routes must be
a=1,c=0,pl=10,as=don't care
These are imported by a border router, which is running BGP to a
stub domain, and not running IBGP to other ASBRs. This causes a
truncation of the AS_PATH. These routes must not be re-exported
into BGP at another ASBR.
4.4.4. Routes with complete path information, pl = 0.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with either
complete path information or are known to be complete through
means other than that carried by the routing protocol.
The OSPF tag to BGP attribute mappings for these routes must be
a=1,c=1,pl=00,as=0 => O=<IGP>, P=<l>
This should be used for importing routes into OSPF from an IGP.
Varadhan [Page 10]RFC 1364 BGP OSPF Interaction September 1992
4.4.5. Routes with complete path information, pl = 1.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|1| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with either
complete path information, or are known to be complete through
means other than that carried by the routing protocol. The
routing protocol also has additional information about the
neighbour AS of the route.
The OSPF tag to BGP attribute mappings for these routes must be
a=1,c=1,pl=01,as=nh => O=<IGP>, P=<l, nh>
This setting should be used when the administrator explicitly
associates an AS number with an instance of an IGP. This setting
can also be used when importing BGP routes whose origin=<IGP> and
AS_PATH=<nh>; if the BGP learned route has no other transitive
attributes, then its propogation via IBGP can be suppressed.
4.4.6. Routes with complete path information, pl >= 1.
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|1|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These are routes imported from routing protocols with complete
path information and carry the AS path information as part of the
routing information.
The OSPF tag must be set to
a=1,c=1,pl=10,as=don't care
These routes must not be exported into BGP because these routes
are already imported from BGP into the OSPF RD. Hence, it is
assumed that the BGP speaker will convey this information to other
BGP speakers via IBGP.
Note that an implementation may import BGP routes with a path
length of 1 and no other transitive attributes directly into OSPF
and not send these routes via IBGP. In this situation, it must
use tag settings corresponding to 4.1.2.2, or 4.1.2.5.
Varadhan [Page 11]RFC 1364 BGP OSPF Interaction September 1992
4.5. Miscellaneous tag settings
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|x|1|1| Reserved for future use |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The value of pl = 3 is reserved during automatic tag generation.
Routers must not generate such a tag when importing routes into the
OSPF routing domain. ASBRs must ignore tags which indicate a pl = 3.
4.6. Summary of the tag sub-field setting
The following table summarises the various combinations of automatic
tag settings for the Completeness and PathLength sub-field of the
OSPF tag and the default behaviour permitted for each setting.
Completeness := 0 | 1 ;
PathLength := 00 | 01 | 10 | 11;
ORIGIN := <INCOMPLETE> | <IGP> | <EGP>;
AS_PATH := valid AS path settings as defined in BGP.
pl = 00 pl = 01 pl = 10 pl = 11
+----------------------------------------------------------------
|
c = 0 | <EGP><l> <EGP><l,nh> never export reserved
c = 1 | <IGP><l> <IGP><l,nh> out of band reserved
|
The "out of band" in the table above implies that OSPF will not be
able to carry everything that BGP needs in its routing
information. Therefore, some other means must be found to carry
this information. In BGP, this is done via IBGP.
5. Setting OSPF Forwarding Address and BGP NEXT_HOP attribute
Forwarding addresses are used to avoid extra hops between multiple
routers that share a common network and that speak different routing
protocols with each other.
Both BGP and OSPF have equivalents of forwarding addresses. In BGP,
the NEXT_HOP attribute is a well-known, mandatory attribute. OSPF
has a Forwarding address field. We will discuss how these are to be
filled in various situations.
Consider the 4 router situation below:
Varadhan [Page 12]RFC 1364 BGP OSPF Interaction September 1992
RT1 and RT2 are in one autonomous system, RT3 and RT4 are in another.
RT1 and RT3 are talking BGP with each other.
RT3 and RT4 are talking OSPF with each other.
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
| | common network
---+-----------------------+--------------------------
<BGP> | |
+-----+ <OSPF> +-----+
| RT3 | | RT4 |
+-----+ +-----+
- Importing network X to OSPF:
Consider an external network X, learnt via BGP from RT1.
RT3 must always fill the OSPF Forwarding Address with the BGP
NEXT_HOP attribute for the route to network X.
- Exporting network Y to BGP:
Consider a network Y, internal to the OSPF routing domain,
RT3's route to network Y is via RT4, and network Y is to be
exported via BGP to RT1.
If network Y is not a subnetted network, RT3 must fill the
NEXT_HOP attribute for network Y with the address of RT4.
This is to avoid requiring packets to take an extra hop
through RT3 when traversing the AS boundary. This is similar
to the concept of indirect neighbour support in EGP [RFC888,
RFC827].
6. Security Considerations
Security considerations are not discussed in this memo.
7. Acknowledgements
I would like to thank Yakov Rekhter, Jeff Honig, John Moy, Tony Li,
and Dennis Ferguson for their help and suggestions in writing this
document, without which I could not have written this document. I
would also like to thank them for giving me the opportunity to write
this document, and putting up with my muddlements through various
phases of this document.
Varadhan [Page 13]RFC 1364 BGP OSPF Interaction September 1992
I would also like to thank the countless number of people from the
OSPF and BGP working groups who have offered numerous suggestions and
comments on the different stages of this document.
8. Bibliography
[RFC827] Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827,
BBN, October 1982.
[RFC888] Seamonson, L., and E. Rosen, "STUB Exterior Gateway
Protocol", RFC 888, BBN, January 1984.
[RFC1058] Hedrick, C., "Routing Information Protocol", RFC 1058,
Rutgers University, June 1988.
[RFC1267] Lougheed, K., and Y. Rekhter, "A Border Gateway
Protocol 3 (BGP-3)", RFC 1267, cisco Systems,
T.J. Watson Research Center, IBM Corp., October 1991.
[RFC1268] Rekhter, Y., and P. Gross, Editors, "Application of the
Border Gateway Protocol in the Internet", RFC 1268,
T.J. Watson Research Center, IBM Corp., ANS, October 1991.
[RFC1247] Moy, J., "The OSPF Specification - Version 2:", RFC 1247,
Proteon, January 1991.
[ROUTE-LEAKING] Almquist, P., "Ruminations on Route Leaking",
Work in Progress.
[NEXT-HOP] Almquist, P., "Ruminations on the Next Hop",
Work in Progress.
9. Author's Address
Kannan Varadhan
Internet Engineer, OARnet
1224 Kinnear Road
Columbus, OH 43212-1136
EMail: kannan@oar.net
Varadhan [Page 14]
