rfc2519.txt

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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].



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        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].





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2. Route Aggregation Framework

   The framework of inter-domain route aggregation we are proposing can
   be summarized as follows:

      - Aggregation from the originating AS

        That is, in its outbound route announcements, each AS aggregates
        the BGP routes originated by itself, by dedicated AS and by
        private-ASs [10].  ("Routes originated by an AS" refers to
        routes which have that AS first in the AS path attribute.  For
        example, routes statically configured and injected into BGP fall
        into this category.)

        This framework does not depend on "proxy aggregation" which
        refers to route aggregation done by an AS other than the
        originating AS.  This preserves the capability of a multi-homed
        site to control the granularity of routing information injected
        into the global routing system. Since proxy aggregation involves
        coordination among multiple organizations, the complexity of
        doing proxy aggregation increases with the number of parties
        involved in the coordination. The complexity, in turn, impacts
        the practicality of proxy aggregation.

        An AS shall always originate via a stable mechanism (e.g.,
        static route configuration) the BGP routes for the large
        aggregates from which it allocates addresses to customers.  This
        ensures that it is safe for its customers to use BGP "no-
        export".

      - Using BGP community "no-export" toward upstream providers

        That is, in its route announcements toward its upstream
        provider, an AS tags the BGP community "no-export" to routes it
        originates that do not need to be propagated beyond its upstream
        provider (e.g., prefixes allocated by the upstream provider).

   This framework is illustrated in Figure 1. A "Tier 1" provider does
   not use "no-export" in its announcement as it does not have an
   upstream provider.  However, it shall aggregate the routes it
   originates in its outbound announcements towards both peer providers
   and customers.  An AS with an upstream provider shall aggregate the
   routes it originates and use "no-export" toward its upstream provider
   for routes that do not need to be propagated beyond its provider's
   AS.   This recursion shall apply to all levels of the routing
   hierarchy.





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                         Tier 1
                    +-- Provider <--+
                    |               |
o aggregates routes |               |  o announces customer routes
  it originates     |               |  o aggregates routes it originates
                    |               ^  o uses "no-export" if appropriate
                    |
                    +---> Tier 2 <--+
                         Provider   |
                    V               |
                    |               |
o aggregates routes |               |  o announces customer routes
  it originates     |               |  o aggregates routes it originates
                    |               |  o uses "no-export" if appropriate
                    |               |
                    |               ^
                    -> Customer AS


                        Figure 1

   This framework scales well as aggregation is done at all levels of
   the routing system.  It is flexible because the originating AS
   controls whether routes of finer granularity are injected to, and/or
   propagated by, its upstream provider.  It facilitates multi-homing
   without compromising route aggregation.

   This framework is detailed in the following sections.

3. Aggregation from the Originating AS

   It has been well recognized that address allocation and address
   renumbering are keys to containing the growth of the Internet routing
   table [1, 2, 8, 9, 11, 12].

   Although the strategies discussed in this document do not assume a
   perfect address allocation, it is strongly urged that an AS receive
   allocation from its upstream service providers' address block.

3.1 Intra-Domain Aggregation

   To reduce the number of routes that need to be injected into an AS,
   there are a couple of principles that shall be followed:








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      - Carry in its BGP table the large route block allocated from its
        upstream provider or an address registry (e.g., InterNIC, RIPE,
        APNIC).  This can be done by either static configuration of the
        large block or by aggregating more specific BGP routes.  The
        former is recommended as it does not depend on other routes.

      - Allocate sub-blocks to the access routers where further
        allocation is done.  That is, the address allocation shall be
        done such that only a few, less specific routes (instead of many
        more, specific ones) need to be known to the other routers
        within the AS.

        For example, a prefix of /17 can be further allocated to
        different access routers as /20s which can then be allocated to
        customers connected to different interfaces on that router (as
        shown in Figure 2).  Then in general only the /20 needs to be
        injected into the whole AS. Exceptions need to be made for
        multi-homed static routes.

                         access router
                        +------------+
                        | x.x.x.x/20 |
                        +------------+
                         |     |    |
                         |     |    |
                         /24   /22  /25


                           Figure 2

   It is noted that rehoming of customers without renumbering even
   within the same AS may lead to injection of more specific routes.
   However, in general the more-specifics do not need to be advertised
   outside of that AS. Such routes can either be tagged with the BGP
   community "no-export" or filtered out by a prefix-based filter to
   prevent them from being advertised out.

3.2 Inter-Domain Aggregation

   There are at least two types of routes that need to be advertised by
   an AS: routes originated by the AS and routes originated by its BGP
   customers.  An AS may need to advertise full routes to certain BGP
   customers, in which case the routing announcements include routes
   originated by non-customer ASs.  Clearly an AS can, and should,
   safely aggregate the routes originated by itself and by its BGP
   customers multi-homed only to it (using, e.g., the dedicated-AS and





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RFC 2519             Inter-Domain Route Aggregation        February 1999


   by the private-AS mechanism [10]) in its outbound announcement.  But
   it is far more dangerous to aggregate routes originated by customer
   ASs due to multi-homing.

   However, there are several cases in which a route originated by a BGP
   customer (other than using the dedicated AS or private AS) does not
   need to be advertised out by its upstream providers.  For example,

      - The route is a more-specific of the upstream provider's block.
        However, the customer is either singly homed; or its connection
        to this particular upstream provider is used for backup only.

      - The more-specifics of a larger block are announced by the
        customer in order to balance traffic over the multiple links to
        the upstream provider.

   Our approach to suppress such routes is to give control to the ASs
   that originate the more-specifics (as seen by its upstream providers)
   and let them tag the BGP community "no-export" to the appropriate
   routes.

   The BGP community "no-export" is a well known BGP community [6, 7].
   A route with this attribute is not propagated beyond an AS boundary.
   So, if a route is tagged with this community in its announcement to
   an upstream provider and is accepted by the upstream provider, the
   route will not be announced beyond the upstream provider's AS. This
   achieves the goal of suppressing the more-specifics in the upstream
   provider's outbound announcement.

   In this framework, the BGP community "no-export" shall be tagged to
   routes that are to be advertized to, but not propagated by, its
   upstream provider.  They may include routes allocated out of its
   upstream provider's block or the more specific routes announced to
   its upstream provider for the purpose of load balancing. This
   aggregation strategy can be implemented via prefix-based filtering as
   shown in the example of Section 5.

   For its own protection, a downstream AS shall announce only its own
   routes and its customer routes to its upstream providers.  Thus, the
   outbound routing announcement and aggregation policy can be expressed
   as follows:

      For routes originated by itself/dedicated-AS/private-AS:
         tag with "no-export" when appropriate, and advertise the
         large block and suppress the more-specifics

      For routes originated by customer ASs:
         advertise to upstream ASs



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      For any other routes:
         do not advertise to upstream ASs

   This approach is flexible and scales well as it gives control to the
   party with the special needs, distributes the workload and avoids the
   coordination overhead required by proxy aggregation.

4. Aggregation by a Provider

   A provider shall aggregate all the routes it originates, as
   documented in Section 3.  The only difference is that the provider
   may be providing full routes to certain BGP customers where no
   outbound filtering is presently in place.  Experience has shown that
   inconsistent route announcement (e.g., aggregate at the interconnects
   but not toward certain customers) can cause serious routing problems
   for the Internet as a whole because of longest-match routing.  In
   certain cases announcing the more-specifics to customers might
   provide for more accurate IGP metrics and could be useful for better
   load-balancing.  However, the potential risk seems to outweigh the
   benefit, especially given the increasing complexity of connectivity
   that a customer may have.  As a result, every effort shall be made to
   ensure consistent route aggregation for all BGP peers.  This means
   deploying filters for the BGP peers which receive full routes.