draft-ietf-isis-wg-255adj-00.txt   draft-ietf-isis-wg-255adj-01.txt 
Internet Engineering Task Force T. Przygienda Internet Engineering Task Force T. Przygienda
INTERNET DRAFT Bell Labs INTERNET DRAFT Bell Labs
1 Nov 1998 1 Jun 1999
Maintaining more than 255 adjacencies in IS-IS Maintaining more than 255 circuits in IS-IS
<draft-ietf-isis-wg-255adj-00.txt> <draft-ietf-isis-wg-255adj-01.txt>
Status of This Memo Status of This Memo
This document is an Internet Draft, and can be found as This document is an Internet-Draft and is in full conformance with
draft-ietf-isis-255adj-00.txt in any standard internet drafts all provisions of Section 10 of RFC2026.
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Abstract Abstract
This draft describes an optional implementation technique within This draft describes an optional implementation technique within
IS-IS [ISO90, Cal90a, Cal90b] used today by several ISPs for routing IS-IS [ISO90, Cal90a, Cal90b] used today by several ISPs for routing
within their clouds. IS-IS is an interior gateway routing protocol within their clouds. IS-IS is an interior gateway routing protocol
developed originally by OSI and used with IP extensions as IGP. developed originally by OSI and used with IP extensions as IGP.
This draft describes how to effectively bypass the problem of 255 This draft describes how to effectively bypass the problem of 255
circuits that IS-IS allows to maintain with minor violations of the circuits that IS-IS allows to maintain with minor violations of the
specification that however does not prevent interoperability with specification that however does not prevent interoperability with
existing implementations. existing implementations.
1. Introduction 1. Introduction
IS-IS reserves within its packets only one byte for a circuit ID IS-IS reserves within its packets only one byte for a circuit ID
and the specification requests it to be unique. This ID is used in and the specification requests it to be unique. This ID is used in
broadcast networks to identify a PNode. They don't seem to serve any broadcast networks to identify a PNode. They don't seem to serve any
particular purpose on p2p networks except for some checking in hellos particular purpose on p2p networks except for some checking in hellos
and tie-breaking in removal of excess adjacencies. and tie-breaking in removal of excess adjacencies.
2. More than 255 adjacencies on p2p links 2. More than 255 circuits for p2p links
For all p2p links within an Intermediate System, the same circuit ID For all p2p links within an Intermediate System, the same circuit ID
can be chosen safely to interact with other Intermediate Systems. can be chosen safely to interact with other Intermediate Systems.
Even when two such systems are brought up on two ends of the link Even when two such systems are brought up on two ends of the link
and both pick the same value, IS-IS specification does not preclude and both pick the same value, IS-IS specification does not preclude
such a configuration. In case of multiple parallel links between the such a configuration. In case of multiple parallel links between the
systems the only obvious problem is the impact on tie-breaking in systems the only obvious problem is the impact on tie-breaking in
case of excessive adjacencies. However, such a configuration cannot case of excessive adjacencies. However, such a configuration cannot
generate forwarding loops. generate forwarding loops.
3. More than 255 adjacencies on broadcast links 3. More than 255 circuits for broadcast links
More trickier a case is the one in which an intermediate system has Trickier a case is the one in which an intermediate system has to
to form adjacencies on a broadcast medium. The decisive insight form adjacencies on a broadcast medium. The decisive insight is the
is the fact that unique circuit ID on a broadcast medium is only fact that unique circuit ID on a broadcast medium is only needed
needed in the case where the given intermediate system is assuming in the case where the given intermediate system is assuming the
the role of the DIS for the LAN. As long as the intermediate system role of the DIS for the LAN. As long as the intermediate system has
has not elected itself DIS, it can use a non-unique circuit ID (1). not elected itself DIS, it can use a non-unique circuit ID (1).
Therefore, it is enough to change circuit ID in all the packets Therefore, it is enough to change circuit ID in all the packets
transmitted to a unique one as soon as DIS election ran and the transmitted to a unique one as soon as DIS election ran and the
system must become DIS. Such behavior is basically indistinguishable system must become DIS. Such behavior is basically indistinguishable
from a node crashing and coming immediately back with a different from a node crashing and coming immediately back with a different
circuit ID than the one used before the crash. Implementation circuit ID than the one used before the crash. Implementation
experience shows that it is unwise to tear down the adjacencies experience shows that it is unwise to tear down the adjacencies by
before changing circuit IDs since this can lead to ``ripple''-down sending empty hellos before changing circuit IDs since this can lead
behavior in DIS property on the broadcast media in case of all to ``ripple''-down behavior in DIS in the case of all routers having
routers having the same preference. the same preference. However, to ensure that all involved parties
agree on LAN ID of the media, implementations must interpret in
subsection 8.4.5 within 10589 the specified condition
f) the set of Intermediate system adjacency states
as including any change in LAN ID to be an indication triggering the
recomputation of the according DIS. Additionally, when recycling a
previously used circuit ID and reusing it on another LAN special
___________________________________________
1. it is important to realize that circuit ID is not a part of
tie-breaking rules on DIS election
care has to be taken that the newly generated pseudonode LSPs have
sequence numbers high enough as to prevent unnecessary flooding.
3.1. Modification of the Behavior on Broadcast Media 3.1. Modification of the Behavior on Broadcast Media
The exact modification of the behavior for broadcast links is The exact modification of the behavior for broadcast links is
given here. It is a modification of chapter 8.4.5 of the original given here. It is a modification of chapter 8.4.5 of the original
specification that states: specification that states:
___________________________________________
1. it is important to realize that circuit ID is not a part of
tie-breaking rules on DIS election
When the broadcast circuit is enabled on an Intermediate system the When the broadcast circuit is enabled on an Intermediate system the
IS shall perform the following actions. IS shall perform the following actions.
a) Commence sending IIH PDUs with the LAN ID field set to the a) Commence sending IIH PDUs with the LAN ID field set to the
concatenation of its own SystemID and its locally assigned one concatenation of its own SystemID and its locally assigned one
octet Local Circuit ID. octet Local Circuit ID.
b) ... <omitted for clarity> b) ... <omitted for clarity>
c) Start listening for ISO 9542 ISH PDUs and ESH PDUs and c) Start listening for ISO 9542 ISH PDUs and ESH PDUs and
acquire adjacencies as appropriate. Do not run the Designated acquire adjacencies as appropriate. Do not run the Designated
Intermediate System election process. Intermediate System election process.
skipping to change at page 4, line 37 skipping to change at page 5, line 5
LAN: LAN:
1) Commence sending IIH PDUs with the LAN ID field set to the 1) Commence sending IIH PDUs with the LAN ID field set to the
concatenation of its own SystemID and a local unique circuit concatenation of its own SystemID and a local unique circuit
ID. ID.
2) go to step b. 2) go to step b.
otherwise exhibit standard behavior. otherwise exhibit standard behavior.
3.2. Multiple levels
Since election of DIS is performed indpendently at each level, the
extensions can be applied to generate local circuit IDs only for the
level at which the system has been elected.
4. Acknowledgments 4. Acknowledgments
Some smart people probably thought about most of these things before Some smart people probably thought about most of these things before
and the p2p case may be documented in the best current practices for and the p2p case may be documented in the best current practices for
IS-IS in the Internet. IS-IS in the Internet. Mike Shand, Tony Li, Arun Satyanarayana,
Rajesh Varadarajan and Christian Hopps provided additional comments.
5. Security Consideration 5. Security Consideration
ISIS security applies to the work presented. No specific security ISIS security applies to the work presented. No specific security
issues with the proposed solutions are known. issues with the proposed solutions are known.
6. Intellectual Property Considerations 6. Intellectual Property Considerations
Lucent may seek patent or other intellectual property protection Lucent may seek patent or other intellectual property protection
for some or all of the technologies disclosed in this document. If for some or all of the technologies disclosed in this document. If
any standards arising from this document are or become protected any standards arising from this document are or become protected
by one or more patents assigned to Lucent, Lucent intends to by one or more patents assigned to Lucent, Lucent intends to
disclose those patents and license them under openly specified and disclose those patents and license them under openly specified and
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