Network Working Group T. Li
Request for Comments: 2966 Procket Networks
Category: Informational T. Przygienda
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
improve the quality of the resulting routes. A well know property of
prefix summarization or any abstraction mechanism is that it
necessarily results in a loss of information. This loss of
information in turn results in the computation of a route based upon
less information, which will frequently result in routes that are not
A simple example can serve to demonstrate this adequately. Suppose
that a L1 area has two L1L2 routers that both advertise a single
summary of all prefixes within the L1 area. To reach a destination
inside the L1 area, any other L2 router is going to compute the
shortest path to one of the two L1L2 routers for that area. Suppose,
for example, that both of the L1L2 routers are equidistant from the
L2 source, and that the L2 source arbitrarily selects one L1L2
router. This router may not be the optimal router when viewed from
the L1 topology. In fact, it may be the case that the path from the
selected L1L2 router to the destination router may traverse the L1L2
router that was not selected. If more detailed topological
information or more detailed metric information was available to the
L2 source router, it could make a more optimal route computation.