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Using a Dedicated AS for Sites Homed to a Single Provider
RFC 2270

Document type: RFC - Informational (January 1998)
Document stream: IETF
Last updated: 2013-03-02
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IESG State: RFC 2270 (Informational)
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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

Stewart, et. al.             Informational                      [Page 1]
RFC 2270                     Dedicated AS                   January 1998

   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.

Stewart, et. al.             Informational                      [Page 2]
RFC 2270                     Dedicated AS                   January 1998

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