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rfc:rfc2519

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:
  1. 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.
  1. 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:
  1. 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.
  1. 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
 be summarized as follows:
  1. 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".
  1. 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.

Chen & Stewart Informational [Page 3] RFC 2519 Inter-Domain Route Aggregation February 1999

                       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:

Chen & Stewart Informational [Page 4] RFC 2519 Inter-Domain Route Aggregation February 1999

  1. 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.
  1. 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

Chen & Stewart Informational [Page 5] 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,
  1. 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.
  1. 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

Chen & Stewart Informational [Page 6] RFC 2519 Inter-Domain Route Aggregation February 1999

    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.
 In summary, the aggregation strategy for a provider shall be:
  1. In announcing customer routes:
      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 other customer ASs:
         advertise
      For any other routes:
         do not advertise
  1. In announcing full routes:
      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 any other routes:
         advertise

Chen & Stewart Informational [Page 7] RFC 2519 Inter-Domain Route Aggregation February 1999

5. An Example

 Consider the example shown in Figure 3 where AS 1000 is a "Tier 1"
 provider with two large aggregates 208.128.0.0/12 and 166.55.0.0/16,
 and AS 2000 is a customer of AS 1000 with a "portable address"
 160.75.0.0/16 and an address 208.128.0.0/19 allocated from AS 1000.
 Assume that 208.128.0.0/19 does not need to be propagated beyond AS
 1000.
                           +----------------+
                           |    AS 1000     |
                           | 208.128.0.0/12 |
                           | 166.55.0.0/16  |
                           +----------------+
                                   |
                                   | BGP
                                   |
                                   |
                           +----------------+
                           |     AS 2000    |
                           | 208.128.0.0/19 |
                           | 160.75.0.0/16  |
                           +----------------+
                                Figure 3
 Then, based on the framework presented, AS 1000 would
  1. originate and advertise the BGP routes 208.128.0.0/12 and

166.55.0.0/16, and suppress more-specifics originated by

      itself/private-ASs/dedicated-ASs
  1. advertise the routes received from the customer AS 2000
 and AS 2000 would
  1. originate BGP route 208.128.0.0/19 and 160.75.0.0/16
  1. advertise both 160.75.0.0/16 and 208.128.0.0/19 to its provider

AS 1000 and suppress the more specifics originated by

      itself/private-AS/dedicated-AS, tagging the route 208.128.0.0/19
      with "no-export"
  1. advertise both 160.75.0.0/16 and 208.128.0.0/19 to its BGP

customers (if any) and suppress the more-specifics originated by

      itself/private-AS/dedicated-AS, plus any other routes the
      customers may desire to receive

Chen & Stewart Informational [Page 8] RFC 2519 Inter-Domain Route Aggregation February 1999

 The sample configuration which implement these policies (in Cisco
 syntax) is given in Appendix A.

6. Acknowledgments

 The authors would like to thank Roy Alcala of MCI for a number of
 interesting hallway discussions related to this work.  The IETF's IDR
 Working Group also provided many helpful comments and suggestions.

7. References

 [1] Rekhter, Y. and T. Li, "An Architecture for IP Address Allocation
     with CIDR", RFC 1518, September 1993.
 [2] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter-
     Domain Routing (CIDR): an Address Assignment and Aggregation
     Strategy", RFC 1519, September 1993.
 [3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
     RFC 1771, March 1995.
 [4] Rekhter, Y. and P., Gross, "Application of the Border Gateway
     Protocol in the Internet", RFC 1772, March 1995.
 [5] Rekhter, Y., "Routing in a Multi-provider Internet", RFC 1787,
     April 1995.
 [6] Chandra, R., Traina, P. and T. Li, "BGP Communities Attribute",
     RFC 1997, August 1996.
 [7] Chen, E. and T. Bates, "An Application of the BGP Community
     Attribute in Multi-home Routing", RFC 1998, August 1996.
 [8] Ferguson, P. and H. Berkowitz, "Network Renumbering Overview: Why
     would I want it and what is it anyway?", RFC 2071, January 1997.
 [9] Berkowitz, H., "Router Renumbering Guide", RFC 2072, January
     1997.
 [10] Stewart, J., Bates, T., Chandra, R., and Chen, E., "Using a
      Dedicated AS for Sites Homed to a Single Provider", RFC 2270,
      January 1998.
 [11] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
      Behaviour Today", RFC 2101, February 1997.
 [12] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
      1900, February 1996.

Chen & Stewart Informational [Page 9] RFC 2519 Inter-Domain Route Aggregation February 1999

 [13] Cisco systems, Cisco IOS Software Version 10.3 Router Products
      Configuration Guide (Addendum), May 1995.

8. Authors' Addresses

 Enke Chen
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA  95134-1706
 Phone: +1 408 527 4652
 EMail: enkechen@cisco.com
 John W. Stewart, III
 Juniper Networks, Inc.
 385 Ravendale Drive
 Mountain View, CA  94043
 Phone: +1 650 526 8000
 EMail: jstewart@juniper.net

Chen & Stewart Informational [Page 10] RFC 2519 Inter-Domain Route Aggregation February 1999

A. Appendix A: Example Cisco Configuration

 This appendix lists the Cisco configurations for AS 2000 of the
 examples presented in Section 5.  The configuration here uses the
 AS-path for outbound filtering although it can also be based on BGP
 community.  Several route-maps are defined that can be used for
 peering with the upstream provider, and for peering with customers
 (announcing full routes or customer routes).

!!# inject aggregates ip route 160.75.0.0 255.255.0.0 Null0 254 ip route 208.128.0.0 255.255.224.0 Null0 254 ! router bgp 2000 network 160.75.0.0 mask 255.255.0.0 network 208.128.0.0 mask 255.255.224.0 neighbor x.x.x.x remote-as 1000 neighbor x.x.x.x route-map export-routes-to-provider out neighbor x.x.x.x send-community ! !!# match all ip as-path access-list 1 permit .* ! !!# List of internal AS and private ASs that are safe to aggregate ip as-path access-list 10 permit ^$ ip as-path access-list 10 permit ^64999_ ip as-path access-list 10 deny .* ! !!# list of other customer ASs ip as-path access-list 20 permit ^3000_

!!# List of prefixes to be tagged with "no-export" access-list 101 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0 !!# Filter out the more specifics of large aggregates, and permit the rest access-list 102 permit ip 160.75.0.0 0.0.0.0 255.255.0.0 0.0.0.0 access-list 102 deny ip 160.75.0.0 0.0.255.255 255.255.128.0 0.0.127.255 access-list 102 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0 access-list 102 deny ip 208.128.0.0 0.0.31.255 255.255.240.0 0.0.16.255 access-list 102 permit ip any any !

!!# route-map with the upstream provider route-map export-routes-to-provider permit 10 match ip address 101 set community no-export route-map export-routes-to-provider permit 20 match as-path 10 match ip address 102

Chen & Stewart Informational [Page 11] RFC 2519 Inter-Domain Route Aggregation February 1999

route-map export-routes-to-provider permit 30 match as-path 20 ! !!# route-map with BGP customers that desire only customer routes route-map export-customer-routes permit 10 match as-path 10 match ip address 102 route-map export-customer-routes permit 20 match as-path 20 ! !!# route-map with BGP customers that desire full routes route-map export-full-routes permit 10 match as-path 10 match ip address 102 route-map export-full-routes permit 20 match as-path 1 !

Chen & Stewart Informational [Page 12] RFC 2519 Inter-Domain Route Aggregation February 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
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 The limited permissions granted above are perpetual and will not be
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 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Chen & Stewart Informational [Page 13]

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