Domain-Wide Prefix Distribution with Two-Level IS-IS
RFC 5302

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Network Working Group                                              T. Li
Request for Comments: 5302                        Redback Networks, Inc.
Obsoletes: 2966                                                  H. Smit
Updates: 1195
Category: Standards Track                                  T. Przygienda
                                                                 Z2 Sagl
                                                            October 2008

          Domain-Wide Prefix Distribution with Two-Level IS-IS

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes extensions to the Intermediate System to
   Intermediate System (IS-IS) protocol to support optimal routing
   within a two-level domain.  The IS-IS protocol is specified in ISO
   10589, with extensions for supporting IPv4 (Internet Protocol)
   specified in RFC 1195.  This document replaces RFC 2966.

   This document extends the semantics presented in RFC 1195 so that a
   routing domain running with both level 1 and level 2 Intermediate
   Systems (IS) (routers) can distribute IP prefixes between level 1 and
   level 2, and vice versa.  This distribution requires certain
   restrictions to ensure that persistent forwarding loops do not form.
   The goal of this domain-wide prefix distribution is to increase the
   granularity of the routing information within the domain.

Li, et al.                  Standards Track                     [Page 1]
RFC 5302            Domain-wide Prefix Distribution         October 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Motivations for Domain-Wide Prefix Distribution  . . . . .  3
     1.2.  Scalability  . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
   2.  Proposed Syntax and Semantics for L2->L1 Inter-Area Routes . .  6
     2.1.  Clarification of External Route-Type and External
           Metric-Type  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Definition of External IP Prefixes in Level 1 LSPs . . . .  8
   3.  Types of IP Routes in IS-IS and Their Order of Preference  . .  8
     3.1.  Overview of All Types of IP Prefixes in IS-IS Link
           State PDUs . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Order of Preference for all Types of IP Routes in IS-IS  . 11
     3.3.  Additional Notes on What Prefixes to Accept or
           Advertise  . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Inter-Operability with Older Implementations . . . . . . . . . 12
   5.  Comparisons with Other Proposals . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 14

Li, et al.                  Standards Track                     [Page 2]
RFC 5302            Domain-wide Prefix Distribution         October 2008

1.  Introduction

   This document describes extensions to the Intermediate System to
   Intermediate System (IS-IS) protocol to support optimal routing
   within a two-level domain.  The IS-IS protocol is specified in
   [ISO-10589], with extensions for supporting IPv4 (Internet Protocol)
   specified in [RFC1195].

   This document replaces [RFC2966], which was an Informational
   document.  This document is on the standards track.  No other
   intentional substantive changes have been made.

   This document extends the semantics presented in RFC 1195 so that a
   routing domain running with both level 1 and level 2 Intermediate
   Systems (IS) (routers) can distribute IP prefixes between level 1 and
   level 2, and vice versa.  This distribution requires certain
   restrictions to ensure that persistent forwarding loops do not form.
   The goal of this domain-wide prefix distribution is to increase the
   granularity of the routing information within the domain.

   An IS-IS routing domain (a.k.a. an autonomous system running IS-IS)
   can be partitioned into multiple level 1 (L1) areas, and a level 2
   (L2) connected subset of the topology that interconnects all of the
   L1 areas.  Within each L1 area, all routers exchange link state
   information.  L2 routers also exchange L2 link state information to
   compute routes between areas.

   RFC 1195 defines the Type, Length, and Value (TLV) tuples that are
   used to transport IPv4 routing information in IS-IS.  RFC 1195 also