Network Working Group J. Parker, Ed.
Request for Comments: 3787 Axiowave Networks
Category: Informational May 2004
Recommendations for Interoperable IP Networks
using Intermediate System to Intermediate System (IS-IS)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document discusses a number of differences between the
Intermediate System to Intermediate System (IS-IS) protocol used to
route IP traffic as described in RFC 1195 and the protocol as it is
deployed today. These differences are discussed as a service to
those implementing, testing, and deploying the IS-IS Protocol to
route IP traffic. A companion document describes the differences
between the protocol described in ISO 10589 and current practice.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 2
3. Unused Features . . . . . . . . . . . . . . . . . . . . . . . 2
4. Overload Bit. . . . . . . . . . . . . . . . . . . . . . . . . 3
5. Migration from Narrow Metrics to Wide . . . . . . . . . . . . 4
6. Intermediate System Hello (ISH) PDU . . . . . . . . . . . . . 6
7. Attached Bit. . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Default Route . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Non-homogeneous Protocol Networks . . . . . . . . . . . . . . 8
10. Adjacency Creation and IP Interface Addressing. . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References. . . . . . . . . . . . . . . . . . 10
12.2. Informative References. . . . . . . . . . . . . . . . . 10
13. Author's Address. . . . . . . . . . . . . . . . . . . . . . . 10
14. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 11
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1. Introduction
Interior Gateway Protocols such as IS-IS are designed to provide
timely information about the best routes in a routing domain. The
original design of IS-IS, as described in ISO 10589 [1] has proved to
be quite durable. However, a number of original design choices have
been modified. This document describes some of the differences
between the protocol as described in RFC 1195 [2] and the protocol
that can be observed on the wire today. A companion document
describes the differences between the protocol described in ISO 10589
and current practice [8].
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are to be interpreted as described in RFC 2119 [3].
2. Acknowledgments
This document is the work of many people, and is the distillation of
over a thousand mail messages. Thanks to Vishwas Manral, who pushed
to create such a document. Thanks to Danny McPherson, the original
editor, for kicking things off. Thanks to Mike Shand, for his work
in creating the protocol, and his uncanny ability to remember what
everything is for. Thanks to Micah Bartell and Philip Christian, who
showed us how to document difference without displaying discord.
Thanks to Les Ginsberg, Neal Castagnoli, Jeff Learman, and Dave Katz,
who spent many hours educating the editor. Thanks to Radia Perlman,
who is always ready to explain anything. Thanks to Satish Dattatri,
who was tenacious in seeing things written up correctly, and to Bryan
Boulton for his work on the IP adjacency issue. Thanks to Russ
White, whose writing improved the treatment of every topic he
touched. Thanks to Shankar Vemulapalli, who read several drafts with
close attention. Thanks to Don Goodspeed, for his close reading of
the text. Thanks to Michael Coyle for identifying the quotation from
Jan L.A. van de Snepscheut. Thanks for Alex Zinin's ministrations
behind the scenes. Thanks to Tony Li and Tony Przygienda, who kept
us on track as the discussions veered into the weeds. And thanks to
all those who have contributed, but whose names I have carelessly
left from this list.
3. Unused Features
Some features defined in RFC 1195 are not in current use.
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3.1. Inter-Domain Routing Protocol Information TLV, Code 131
RFC 1195 defines an Inter-Domain Routing Protocol Information TLV,
with code 131, designed to convey information transparently between
boundary routers. TLV 131 is not used, and MUST be ignored if
received.
3.2. Authentication TLV, Code 133
RFC 1195 defines an authentication TLV, code 133, which contains
information used to authenticate the PDU. This TLV has been replaced
by TLV 10, described in "IS-IS Cryptographic Authentication" [4].
TLV 133 is not used, and MUST be ignored.
4. Overload Bit
To deal with transient problems that prevent an IS from storing all
the LSPs it receives, ISO 10589 defines an LSP Database Overload
condition in section 7.3.19. When an IS is in Database Overload
condition, it sets a flag called the Overload Bit in the non-
pseudonode LSP number Zero that it generates. Section 7.2.8.1 of ISO
10589 instructs other systems not to use the overloaded IS as a
transit router. Since the overloaded IS does not have complete
information, it may not be able to compute the right routes, and
routing loops could develop. However, an overloaded router may be
used to reach End Systems directly attached to the router, as it may
provide the only path to an End System.
The ability to signal reduced knowledge is so useful that the meaning
of this flag has been overloaded. In a Service Provider's network,
when a router running BGP and IS-IS reboots, BGP might take more time
to converge than IS-IS. Thus the router may drop traffic for
destinations not yet learned via BGP. It is convenient to set the
Overload Bit until BGP has converged, as described in "Intermediate
System to Intermediate System (IS-IS) Transient Blackhole Avoidance"
[6].
An implementation SHOULD use the Overload Bit to signal that it is
not ready to accept transit traffic.
An implementation SHOULD not set the Overload bit in PseudoNode LSPs
that it generates, and Overload bits seen in PseudoNode LSPs SHOULD
be ignored. This is also discussed in the companion document on ISO
interoperability [8].
RFC 1195 makes clear when describing the SPF algorithm for IP routers
in section C.1.4 that directly connected IP subnetworks are reachable
when an IS is overloaded.
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Note that the End Systems neighbors of the system P includes IP
reachable address entries included in the LSPs from system P.
When processing LSPs received from a router which has the Overload
bit set in LSP number Zero, the receiving router SHOULD treat all IP
reachability advertisements as directly connected and use them in its
SPF computation.
Since the IP prefixes that an overloaded router announces will be
treated as directly attached, an overloaded router SHOULD take care
in selecting which routes to advertise in the LSPs it generates.
5. Migration from Narrow Metrics to Wide
The IS-Neighbors TLV (TLV 2) as defined in ISO 10589 and the IP
Reachability TLV (TLV 128/TLV 130) as defined in RFC 1195 provide a 6
bit metric for the default link metric to the listed neighbor. This
metric has proved too limited. The Extended IS-Neighbors TLV (TLV
22) and the Extended IP Reachability TLV (TLV 135) are defined in
"IS-IS extensions for Traffic Engineering" [5]. The Extended IS-
Neighbors TLV (TLV 22) defines a 24 bit metric, and the Extended IP
Reachability TLV (TLV 135) defines a 32 bit metric for IP Networks
and Hosts.
If not all devices in the IS-IS domain support wide metrics, narrow
metrics MUST continue to be used. Once all devices in the network
are able to support the new TLVs containing wide metrics, the network
can be migrated to the new metric style, though care must be taken to
avoid routing loops.
We make the following assumptions about the implementation:
(1) Each system can generate and understand both narrow and wide
metrics.
(2) The implementation can run the SPF algorithm on an LSP DB
with instances of both metric styles.
(3) If there are two metric styles for a link or IP prefix, it
will pick one of them as the true cost for the link.
To compare the different variants of the narrow metric with wide
metrics, we need an algorithm that translates External and Internal
narrow metrics into a common integer range. Since we have different
computations for the L1 and L2 routes, we only need to map metrics
from a single level.
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In RFC 1195 section 3.10.2, item 2c) states that the IP prefixes
located in "IP External Reachability" with internal-metric and IP
prefixes located in "IP Internal Reachability" with internal-metric
have the same preference. As defined in "Domain-wide Prefix
Distribution with Two-Level IS-IS", the Most Significant Bit on an L1
metric tells us if the route has been leaked down, but does not
change the distance. Thus we will ignore the MSBit.
We interpret the default metric as an 7 bit quantity. Metrics with
the external bit set are interpreted as metrics in the range
[64..127]. Metrics with the external bit clear are interpreted as
metrics in the range [0..63].
5.1. Transition Algorithm
To facilitate a smooth transition between the use of narrow metrics
exclusively to the use of wide metrics exclusively, the following
steps must be taken, in the order below.
(1) All routers advertise Narrow Metrics as defined in ISO
10589, and consider narrow metrics only in their SPF
computation.
(2) Each system is configured in turn to send wide metrics as
well as narrow metrics. The two metrics for the same link
or IP prefix SHOULD agree.
(3) When all systems are advertising wide metrics, make any
changes necessary on each system to consider Wide Metrics
during the SPF, and change MaxPathMetric to 0xFE000000.
(4) Each system is configured in turn to stop advertising narrow
metrics.
(5) When the network is only using wide metrics, metrics on
individual links may be rescaled to take advantage of the
larger metric.
5.2. Dealing with Non-Equal Metrics
The algorithm above assumes that the metrics are equal, and thus
needs to make no assumption about which metric the SPF algorithm
uses. This section describes the changes that should be made to the
SPF algorithm when both Narrow and Wide metric styles should be
considered. Using a common algorithm allows different
implementations to compute the same distances independently, even if
the wide and narrow metrics do not agree.
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The standard SPF algorithm proceeds by comparing sums of link costs
to obtain a minimal cost path. During transition, there will be more
than one description of the same links. We resolve this by selecting
the minimum metric for each link. This may give us a path with some
links chosen due to a wide metric and some links chosen due to a
narrow metric.
The description below is more complex than the implementation needs
to be: the implementation may simply select the minimal cost neighbor
in TENT, discarding paths to destinations we have already reached, as
described in ISO 10589.
The variables MaxPathMetric and MaxLinkMetric SHOULD retain the
values defined in Table 2 of section 8 of ISO 10589.
In C.2.5 Step 0 of the description of the SPF algorithm, section b)
d(N) = cost of the parent circuit of the adjacency N
If multiple styles of metric for the link are defined, the cost
will be the minimum available cost for the circuit.
In C.2.5 Step 0 of the description of the SPF algorithm, section i)
d(N) = metric of the circuit
If multiple styles of metric for the link are defined, the cost
will be the minimum available cost for the circuit.
In C.2.6 Step 1 of the description of the SPF algorithm, section a)
dist(P,N) = d(P) + metric(P,N)
If multiple styles of metric for the neighbor are defined, the
cost will be the minimum available cost for the circuit.
6. Intermediate System Hello (ISH) PDU
The original intent of RFC 1195 was to provide a routing protocol
capable of handling both CLNS and IPv4 reachability information. To
allow CLNS Endstations (ES) to know that they are attached to a
router, Intermediate Systems are required to send Intermediate System
Hello PDUs (ISH) for End Stations when a point-to-point circuit comes
up. Furthermore, an IS is not allowed to send Intermediate System to
Intermediate System Hello PDUs (IIH) before receiving an ISH from a
peer. This reduces routing protocol traffic on links with a single
IS.
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For this reason section 5.1 RFC 1195 states:
"On point-to-point links, the exchange of ISO 9542 ISHs
(intermediate system Hellos) is used to initialize the link,
and to allow each router to know if there is a router on the
other end of the link, before IS-IS Hellos are exchanged. All
routers implementing IS-IS (whether IP-only, OSI-only, or
dual), if they have any interfaces on point-to-point links,
must therefore be able to transmit ISO 9542 ISHs on their
point-to-point links."
Section 5.1 RFC 1195 reinforces the need to comply with section 8.2.4
of ISO 10589. However, in an IP Only environment, the original need
for the ISH PDU is not present.
A multi-protocol IS that supports the attachment of CLNS ESs over
Point to Point circuits must act in accordance with section 8.2.2 ISO
10589 when CLNS functionality is enabled.
An IP only implementation SHOULD issue an ISH PDU as described in
section 8.2.3 of ISO 10589. This is to inter-operate with
implementations which require an ISH to initiate the formation of an
IS-IS adjacency.
An IP Only implementation may issue an IIH PDU when a point to point
circuit transitions into an "Up" state to initiate the formation of
an IS-IS adjacency, without sending an ISH PDU. However, this may
not inter-operate with implementations which require an ISH for
adjacency formation.
An IS may issue an IIH PDU in response to the receipt of an IIH PDU
in accordance with section 8.2.5.2 ISO 10589, even though it has not
received an ISH PDU.
7. The Attached Bit
In section 7.2.9.2 of ISO 10589, an algorithm is described to
determining when the attachedFlag should be set on an intermediate
system. Some implementations also allow the attachedFlag to be set
on Intermediate Systems routing IP traffic when there is a default
route in the local routing table, or when some other state is reached
that implies a connection to the rest of the network.
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8. Default Route
RFC 1195 states in section 1.3:
Default routes are permitted only at level 2 as external routes
(i.e., included in the "IP External Reachability Information"
field, as explained in sections 3 and 5). Default routes are
not permitted at level 1.
Because of the utility of the default route when dealing with other
routing protocols and the ability to influence the exit point from an
area, an implementation MAY generate default routes in Level 1.
9. Non-homogeneous Protocol Networks
RFC 1195 assumes that every deployment of IS-IS routers will support
a homogeneous set of protocols. It anticipates OSI only, IP only, or
dual OSI and IP routers. While it allows mixed areas with, for
example, both pure IP and Dual IP and OSI routers, it allows only IP
traffic in such domains, and OSI traffic only when pure OSI and Dual
IP and OSI routers are present. Thus it provides only lowest common
denominator routing.
RFC 1195 also requires the inclusion of the Protocol Supported TLV
with code 129 in IIH and ISH PDUs and in LSP number Zero. IP capable
routers MUST generate a Protocol Supported TLV, and MUST include the
IP protocol as a supported protocol. A router that does not include
the Protocols Supported TLV may be assumed to be a pure OSI router
and can be interpreted as implicitly "advertising" support for the
OSI protocol.
The requirements of RFC 1195 are ample if networks adhere to this
restriction. However, the behavior of mixed networks that do not
follow these guidelines is not well defined.
The ITU-T requires that SONET/SDH equipment running the IS-IS
protocol must not form an adjacency with a neighbour unless they
share at least one network layer protocol in common. Unless this
feature is present in every IS in the SONET or SDH DCN network the
network may not function correctly. Implementors MAY include this
feature if they wish to ensure interoperability with SONET and SDH
DCN networks.
Definition of an interoperable strategy for resolving the problems
that arise in non-homogeneous protocol networks remains incomplete.
Members of the ITU are actively working on a proposal: see
"Architecture and Specification of Data Communication Network", [7].
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10. Adjacency Creation and IP Interface Addressing
RFC 1195 states that adjacencies are formed without regard to IP
interface addressing. However, many current implementations refuse
adjacencies based on interface addresses and related issues.
In section 4.2, RFC 1195 requires routers with IP interface addresses
to advertise the addresses in an IP Interface Address TLV (132)
carried in IIH PDUs. Some implementations will not interoperate with
a neighbor router that does not include the IP Interface Address TLV.
Further, some implementations will not form an adjacency on broadcast
interfaces with a peer who does not share an interface address in
some common IP subnetwork.
If a LAN contains a mixture of implementations, some that form
adjacencies with all neighbors and some that do not, care must be
taken when assigning IP addresses. If not all routers in a LAN are
on the same IP subnet, it is possible that DIS election may fail,
leading to the election of multiple DISs on a LAN, or no DIS at all.
Even if DIS election succeeds, black holes can result because the
IS-IS LAN transitivity requirements of section 6.7.3 ISO 10589 are
not met.
Unnumbered point to point links do not have IP interface addresses,
though they may have other IP addresses assigned to the routers. The
IP address assigned to two routers that are neighbors on an
unnumbered point to point link do not need to be related. However,
some implementations will not form an adjacency on numbered point to
point links if the interface addresses of each endpoint are not in
the same IP subnetwork. This means that care must be taken in
assigning IP interface addresses in all networks.
For an implementation to interoperate in a such mixed environment, it
MUST include an IP Interface address (TLV 132) in its IIH PDUs. The
network administrator should ensure that there is a common IP subnet
assigned to links with numbered interfaces, and that all routers on
each link have a IP Interface Addresses belonging to the assigned
subnet.
11. Security Considerations
The clarifications in this document do not raise any new security
concerns, as there is no change in the underlying protocol described
in ISO 10589 [1] and RFC 1195 [2].
The document does make clear that TLV 133 has been deprecated and
replaced with TLV 10.
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12. References
12.1. Normative References
[1] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002.
[2] Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
December 1990.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Li, T. and R. Atkinson, "IS-IS Cryptographic Authentication",
RFC 3567, July 2003.
[5] Smit, H. and T. Li, "Intermediate System to Intermediate System
(IS-IS) Extensions for Traffic Engineering (TE)", RFC 3784, May
2004.
[6] McPherson, D., "Intermediate System to Intermediate System (IS-
IS) Transient Blackhole Avoidance", RFC 3277, April 2002.
12.2. Informative References
[7] ITU, "Architecture and Specification of Data Communication
Network", ITU-T Recommendation G.7712/Y.1703, November 2001
[8] Parker, J., Ed., "Recommendations for Interoperable Networks
using Intermediate System to Intermediate System (IS-IS)", RFC
3719, February 2004.
13. Author's Address
Jeff Parker
Axiowave Networks
200 Nickerson Road
Marlborough, Mass 01752
USA
EMail: jparker@axiowave.com
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14. Full Copyright Statement
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