A Framework for Inter-Domain Route Aggregation
RFC 2519

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Network Working Group                                            E. Chen
Request for Comments: 2519                                         Cisco
Category: Informational                                       J. Stewart
                                                                 Juniper
                                                           February 1999

             A Framework for Inter-Domain Route Aggregation

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 (1999).  All Rights Reserved.

Abstract

   This document presents a framework for inter-domain route aggregation
   and shows an example router configuration which 'implements' this
   framework.  This framework is flexible and scales well as it
   emphasizes the philosophy of aggregation by the source, both within
   routing domains as well as towards upstream providers, and it also
   strongly encourages the use of the 'no-export' BGP community to
   balance the provider-subscriber need for more granular routing
   information with the Internet's need for scalable inter-domain
   routing.

1. Introduction

   The need for route aggregation has long been recognized.  Route
   aggregation is good as it reduces the size, and slows the growth, of
   the Internet routing table.  Thus, the amount of resources (e.g., CPU
   and memory) required to process routing information is reduced and
   route calculation is sped up.  Another benefit of route aggregation
   is that route flaps are limited in number, frequency and scope, which
   saves resources and makes the global Internet routing system more
   stable.

   Since CIDR (Classless Inter-Domain Routing) [2] was introduced,
   significant progress has been made on route aggregation, particularly
   in the following two areas:

      - Formulation and implementation of IP address allocation policies
        by the top registries that conform to the CIDR principles [1].

Chen & Stewart               Informational                      [Page 1]
RFC 2519             Inter-Domain Route Aggregation        February 1999

        This policy work is the cornerstone which makes efficient route
        aggregation technically possible.

      - Route aggregation by large (especially "Tier 1") providers.  To
        date, the largest reductions in the size of the routing table
        have resulted from efficient aggregation by large providers.

   However, the ability of various levels of the global routing system
   to implement efficient aggregation schemes varies widely.  As a
   result, the size and growth rate of the Internet routing table, as
   well as the associated route computation required, remain major
   issues today.  To support Internet growth, it is important to
   maximize the efficiency of aggregation at all levels in the routing
   system.

   Because of the current size of the routing system and its dynamic
   nature, the first step towards this goal is to establish a clearly
   defined framework in which scaleable inter-domain route aggregation
   can be realized.  The framework described in this document is based
   on the predominant and current experience in the Internet. It
   emphasizes the philosophy of aggregation by the source, both within
   routing domains as well as towards upstream providers.  The framework
   also strongly encourages the use of the "no-export" BGP community to
   balance the providersubscriber need for more granular routing
   information with the Internet's need for scalable inter-domain
   routing.  The advantages of this framework include the following:

      - Route aggregation is done in a distributed fashion, with
        emphasis on aggregation by the party or parties injecting the
        aggregatable routing information into the global mesh.

      - The flexibility of a routing domain to be able to inject more
        granular routing information to an adjacent domain to control
        the resulting traffic patterns, without having an impact on the
        global routing system.

        In addition to describing the philosophy, we illustrate it by
        presenting sample configurations.  IPv4 prefixes, BGP4 and ASs
        are used in examples, though the principles are applicable to
        inter-domain route aggregation in general.

        Address allocation policies and technologies to renumber entire
        networks, while very relevant to the realization of successful
        and sustained inter-domain routing, are not the focus of this
        document.  The references section contains pointers to relevant
        documents [8, 9, 11, 12].

Chen & Stewart               Informational                      [Page 2]
RFC 2519             Inter-Domain Route Aggregation        February 1999

2. Route Aggregation Framework

   The framework of inter-domain route aggregation we are proposing can