Extending the Number of Intermediate System to Intermediate System (IS-IS) Link State PDU (LSP) Fragments Beyond the 256 Limit
RFC 3786

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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

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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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RFC 3786                  IS-IS LSP Fragments                   May 2004

      Normal system-id
         The system-id of an Originating System.

      Additional system-id
         An Additional system-id that is assigned by the network
         administrator.  Each Additional system-id allows generation of
         256 additional, or extended, LSP fragments.  The Additional