Recommendations for Interoperable Networks using Intermediate System to Intermediate System (IS-IS)
RFC 3719

Network Working Group                                     J. Parker, Ed.
Request for Comments: 3719                             Axiowave Networks
Category: Informational                                    February 2004

           Recommendations for Interoperable 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 as
   described in ISO 10589 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.  A companion document
   discusses differences between the protocol described in RFC 1195 and
   the protocol as it is deployed today for routing IP traffic.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Constants That Are Variable . . . . . . . . . . . . . . . . .  2
   3.  Variables That Are Constant . . . . . . . . . . . . . . . . .  4
   4.  Alternative Metrics . . . . . . . . . . . . . . . . . . . . .  6
   5.  ReceiveLSPBufferSize. . . . . . . . . . . . . . . . . . . . .  6
   6.  Padding Hello PDUs. . . . . . . . . . . . . . . . . . . . . .  8
   7.  Zero Checksum . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Purging Corrupted LSPs. . . . . . . . . . . . . . . . . . . . 10
   9.  Checking System ID in Received point-to-point IIH PDUs. . . . 10
   10. Doppelganger LSPs . . . . . . . . . . . . . . . . . . . . . . 11
   11. Generating a Complete Set of CSNPs. . . . . . . . . . . . . . 11
   12. Overload Bit. . . . . . . . . . . . . . . . . . . . . . . . . 12
   13. Security Considerations . . . . . . . . . . . . . . . . . . . 13
   14. References. . . . . . . . . . . . . . . . . . . . . . . . . . 13
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
   16. Author's  Address . . . . . . . . . . . . . . . . . . . . . . 14
   17. Full Copyright Statement. . . . . . . . . . . . . . . . . . . 15

Parker                       Informational                      [Page 1]
RFC 3719           Interoperable Networks using IS-IS      February 2004

1.  Introduction

         In theory, there is no difference between theory and practice.
         But in practice, there is.
                                    Jan L.A. van de Snepscheut

   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 addresses differences between the
   protocol described in ISO 10589 and the protocol that can be observed
   on the wire today.  A companion document discusses differences
   between the protocol described in RFC 1195 [2] for routing IP traffic
   and current practice.

   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.  Constants That Are Variable

   Some parameters that were defined as constant in ISO 10589 are
   modified in practice.  These include the following

         (1)  MaxAge - the lifetime of a Link State PDU (LSP)

         (2)  ISISHoldingMultiplier - a parameter used to describe the
              generation of hello packets

         (3)  ReceiveLSPBufferSize - discussed in a later section

2.1.  MaxAge

   Each LSP contains a RemainingLifetime field which is initially set to
   the MaxAge value on the generating IS.  The value stored in this
   field is decremented to mark the passage of time and the number of
   times it has been forwarded.  When the value of a foreign LSP becomes
   0, an IS initiates a purging process which will flush the LSP from
   the network.  This ensures that corrupted or otherwise invalid LSPs
   do not remain in the network indefinitely.  The rate at which LSPs
   are regenerated by the originating IS is determined by the value of
   maximumLSPGenerationInterval.

Parker                       Informational                      [Page 2]
RFC 3719           Interoperable Networks using IS-IS      February 2004

   MaxAge is defined in ISO 10589 as an Architectural constant of 20
   minutes, and it is recommended that maximumLSPGenerationInterval be