Domain-wide Prefix Distribution with Two-Level IS-IS (RFC2966)
Original Publication Date: 2000-Oct-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
T. Li: AUTHOR [+2]
This document describes extensions to the Intermediate System to Intermediate System (IS-IS) protocol to support optimal routing within a two-level domain. This memo provides information for the Internet community.
Network Working Group T. Li Request for Comments: 2966 Procket Networks Category: Informational T. Przygienda Redback H. Smit Procket Networks October 2000
Domain-wide Prefix Distribution with Two-Level 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 (C) The Internet Society (2000). All Rights Reserved.
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 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 insure 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.
Informational [Page 1]
RFC 2966 Domain-wide Prefix Distribution October 2000
RFC 1195  defines the Type, Length and Value (TLV) tuples that are used to transport IPv4 routing information in IS-IS. RFC 1195 also specifies the semantics and procedures for interactions between levels. Specifically, routers in a L1 area will exchange information within the L1 area. For IP destinations not found in the prefixes in the L1 database, the L1 router should forward packets to the nearest router that is in both L1 and L2 (i.e., an L1L2 router) with the "attached bit" set in its L1 Link State Protocol Data Unit (LSP).
Also per RFC 1195, an L1L2 router should be manually configured with a set of prefixes that summarizes the IP prefixes reachable in that L1 area. These summaries are injected into L2. RFC 1195 specifies no further interactions between L1 and L2 for IPv4 prefixes.
1.1 Motivations for domain-wide prefix distribution
The mechanisms specified in RFC 1195 are appropriate in many situations, and lead to excellent scalability properties. However, in certain circumstances, the domain administrator may wish to sacrifice some amount of scalability and distribute more specific information than is described by RFC 1195. This section discusses the various reasons why the domain administrator may wish to make such a tradeoff.
One major reason for distributing more prefix information is to...