





Network Working Group                                          J. Stewart
Request for Comments: 2270                                            ISI
Category: Informational                                          T. Bates
                                                               R. Chandra
                                                                  E. Chen
                                                                    Cisco
                                                             January 1998


       Using a Dedicated AS for Sites  Homed to a Single Provider

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 (1998).  All Rights Reserved.

Abstract

   With the increased growth of the Internet, the number of customers
   using BGP4 has grown significantly. RFC1930 outlines a set of
   guidelines for when one needs and should use an AS. However, the
   customer and service provider (ISP) are left with a problem as a
   result of this in that while there is no need for an allocated AS
   under the guidelines, certain conditions make the use of BGP4 a very
   pragmatic and perhaps only way to connect a customer homed to a
   single ISP.  This paper proposes a solution to this problem in line
   with recommendations set forth in RFC1930.

1.  Problems

   With the increased growth of the Internet, the number of customers
   using BGP4 [1],[2] has grown significantly. RFC1930 [4] outlines a
   set of guidelines for when one needs and should use an AS. However,
   the customer and service provider (ISP) are left with a problem as a
   result of this in that while there is no need for an allocated AS
   under the guidelines, certain conditions make the use of BGP4 a very
   pragmatic and perhaps only way to connect a customer homed to a
   single ISP. These conditions are as follows:

   1) Customers multi-homed to single provider






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   Consider the scenario outlined in Figure 1 below.



                        +-------+      +-------+
                           +----+       |      |       |
                +------+   |    | ISP A +------+ ISP B |
                | Cust.+---+    |       |      |       |
                |   X  +--------+       |      |       |
                +------+        ++-----++\     +-------+
                                 |     |  \
                                 |     |   \  +--------+
                                ++-----++   +-|        |
                                | Cust. |     |  ISP C |
                                |   Y   |     |        |
                                +-------+     +--------+

          Figure 1: Customers multi-home to a single provider

   Here both customer X and customer Y are multi-homed to a single
   provider, ISP A. Because these multiple connections are "localized"
   between the ISP A and its customers, the rest of the routing system
   (ISP B and ISP C in this case) doesn't need to see routing
   information for a single multi-homed customer any differently than a
   singly-homed customer as it has the same routing policy as ISP A
   relative to ISP B and ISP C.  In other words, with respect to the
   rest of the Internet routing system the organization is singly-homed,
   so the complexity of the multiple connections is not relevant in a
   global sense.  Autonomous System Numbers (AS) are identifiers used in
   routing protocols and are needed by routing domains as part of the
   global routing system.  However, as [4] correctly outlines,
   organizations with the same routing policy as their upstream provider
   do not need an AS.

   Despite this fact, a problem exists in that many ISPs can only
   support the load-sharing and reliability requirements of a multi-
   homed customer if that customer exchanges routing information using
   BGP-4 which does require an AS as part of the protocol.

   2) Singly-homed customers requiring dynamic advertisement of NLRI's

      While this is not a common case as static routing is generally
      used for this purpose, if a large amount of NLRI's need to be
      advertised from the customer to the ISP it is often
      administratively easier for these prefixes to be advertised using
      a dynamic routing protocol. Today, the only exterior gateway
      protocol (EGP) that is able to do this is BGP. This leads to the
      same problem outlined in condition 1 above.



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   As can be seen there is clearly a problem with the recommendations
   set forth in [4] and the practice of using BGP4 in the scenarios
   above. Section 2 proposes a solution to this problem with following
   sections describing the implications and application of the proposed
   solution.

   It should also be noted that if a customer is multi-homed to more
   than one ISP then they are advised to obtain an official allocated AS
   from their allocation registry.

2.  Solution

   The solution we are proposing is that all BGP customers homed to the
   same single ISP use a single, dedicated AS specified by the ISP.

   Logically, this solution results in an ISP having many peers with the
   same AS, although that AS exists in "islands" completely disconnected
   from one another.

   Several practical implications of this solution are discussed in the
   next section.

3. Implications

3.1 Full Routing Table Announcement

   The solution precludes the ability for a BGP customer using the
   dedicated AS to receive 100% full routes.  Because of routing loop
   detection of AS path, a BGP speaker rejects routes with its own AS
   number in the AS path.  Imagine Customer X and Customer Y maintain
   BGP peers with Provider A using AS number N. Then, Customer X will
   not be able to received routes of Customer Y.  We do not believe that
   this would cause a problem for Customer X, though, because Customer X
   and Customer Y are both stub networks so default routing is adequate,
   and the absence of a very small portion of the full routing table is
   unlikely to have a noticeable impact on traffic patterns guided by
   MEDs received.

   A BGP customer using the dedicated AS must carry a default route
   (preferably receiving from its provider via BGP).

3.2  Change of External Connectivity

   The dedicated AS specified by a provider is purely for use in peering
   between its customers and the provider. When a customer using the
   dedicated AS changes its external connectivity, it may be necessary
   for the customer to reconfigure their network to use a different AS
   number (either a globally unique one if homed to multiple providers,



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   or a dedicated AS of a different provider).

3.3  Aggregation

   As BGP customers using this dedicated AS are only homed to one ISP,
   their routes allocated from its providers CIDR block do not need to
   be announced upstream by its provider as the providers will already
   be originating the larger block. [6].

3.4  Routing Registries

   The Internet Routing Registry (IRR) [5] is used by providers to
   generate route filtering lists.  Such lists are derived primarily
   from the "origin" attribute of the route objects.  The "origin" is
   the AS that originates the route.  With multiple customers using the
   same AS, finer granularity will be necessary to generate the correct
   route filtering.  For example, the "mntner" attribute or the
   "community" attribute of a route object can be used along with the
   "origin" attribute in generating the filtering lists.

4. Practice

   The AS number specified by a provider can either be an AS from the
   private AS space (64512 - 65535) [4], or be an AS previously
   allocated to the provider.  With the former, the dedicated AS like
   all other private AS's should be stripped from its AS path while the
   route is being propagated to the rest of the Internet routing system.

5.  Security Considerations

   The usage of AS numbers described in this document has no effective
   security impact.  Acceptance and filtering of AS numbers from
   customers is an issue dealt with in other documents.

6.  Acknowledgments

   The authors would like to thank Roy Alcala of MCI and Arpakorn
   Boonkongchuen for their input to this document.  The members of the
   IDR Working Group also provided helpful comments.

7.  References

   [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
   RFC 1771, March 1995.

   [2] Rekhter, Y., and P. Gross, "Application of the Border Gateway
   Protocol in the Internet", RFC 1772, March 1995.




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   [3] Rekhter, Y., "Routing in a Multi-provider Internet", RFC 1787,
   April 1995.

   [4] Hawkinson, J., and T. Bates, "Guidelines for creation, selection,
   and registration of an Autonomous System (AS)", RFC 1930, March 1996.

   [5] Bates, T., Gerich, E., Joncheray, L., Jouanigot, J-M, Karrenberg,
   D., Terpstra, M., and J. Yu., "Representation of IP Routing Policies
   in a Routing Registry (ripe-81++)", RFC 1786, March 1995.

   [6] Chen, E., and J. Stewart., "A Framework for Inter-Domain Route
   Aggregation", Work in Progress.

8.  Authors' Addresses

   John Stewart
   USC/ISI
   4350 North Fairfax Drive
   Suite 620
   Arlington, VA  22203

   EMail: jstewart@isi.edu


   Tony Bates
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: tbates@cisco.com


   Ravi Chandra
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: rchandra@cisco.com


   Enke Chen
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: enkechen@cisco.com





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9.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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