Network Working Group A. Hermelin
Request for Comments: 3786 Montilio Inc.
Category: Informational S. Previdi
M. Shand
Cisco Systems
May 2004
Extending the Number of
Intermediate System to Intermediate System (IS-IS)
Link State PDU (LSP) Fragments Beyond the 256 Limit
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 describes a mechanism that allows a system to originate
more than 256 Link State PDU (LSP) fragments, a limit set by the
original Intermediate System to Intermediate System (IS-IS) Routing
protocol, as described in ISO/IEC 10589. This mechanism can be used
in IP-only, OSI-only, and dual routers.
Table of Contents
1. Introduction ................................................. 2
1.1. Keywords ............................................... 2
1.2. Definitions of Commonly Used Terms ..................... 2
1.3. Operation Modes ........................................ 3
1.4. Overview ............................................... 4
2. IS Alias ID TLV (IS-A) ....................................... 5
3. Generating LSPs .............................................. 6
3.1. Both Operation Modes ................................... 6
3.2. Operation Mode 1 Additives ............................. 8
4. Purging Extended LSP Fragments ............................... 10
5. Modifications to LSP handling in SPF ......................... 10
6. Forming Adjacencies .......................................... 11
7. Interoperating between extension-capable and non-capable ISs . 11
8. Security Considerations ...................................... 12
9. Acknowledgements ............................................. 12
Hermelin, et al. Informational [Page 1]RFC 3786 IS-IS LSP Fragments May 2004
10. References ................................................... 12
11. Authors' Addresses ........................................... 13
12. Full Copyright Statement ..................................... 14
1. Introduction
In the Intermediate System to Intermediate System (IS-IS) protocol, a
system floods its link-state information in Link State PDU (LSP) Data
Units, or LSPs for short. These logical LSPs can become quite large,
therefore the protocol specifies a means of fragmenting this
information into multiple LSP fragments. The number of fragments a
system can generate is limited by ISO/IEC 10589 [ISIS-ISO] to 256
fragments, where each fragment's size is also limited. Hence, there
is a limit on the amount of link-state information a system can
generate.
A number of factors can contribute to exceeding this limit:
- Introduction of new TLVs and sub-TLVs to be included in LSPs.
- The use of LSPs to propagate various types of information (such as
traffic-engineering information).
- The increasing number of destinations and AS topologies.
- Finer granularity routing, and the ability to inject external
routes into areas [DOMAIN-WIDE].
- Other emerging technologies, such as optical, IPv6, etc.
This document describes mechanisms to relax the limit on the number
of LSP fragments.
1.1. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[BCP14].
1.2. Definitions of Commonly Used Terms
This section provides definitions for terms that are used throughout
the text.
Originating System
A router physically running the IS-IS protocol. As this
document describes methods allowing a single IS-IS process to
advertise its LSPs as multiple "virtual" routers, the
Originating System represents the single "physical" IS-IS
process.
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