Guidelines for creation, selection, and registration of an Autonomous System (AS)
RFC 1930

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Network Working Group                                       J. Hawkinson
Request for Comments: 1930                                    BBN Planet
BCP: 6                                                          T. Bates
Category: Best Current Practice                                      MCI
                                                              March 1996

          Guidelines for creation, selection, and registration
                      of an Autonomous System (AS)

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

Table of Contents

   1. Introduction ............................................    2
   2. Motivation ..............................................    2
   3. Definitions .............................................    2
   4. Common errors in allocating ASes ........................    5
   5. Criteria for the decision -- do I need an AS?  ..........    5
   5.1 Sample Cases ...........................................    6
   5.2 Other Factors ..........................................    7
   6. Speculation .............................................    7
   7. One prefix, one origin AS ...............................    8
   8. IGP issues ..............................................    8
   9. AS Space exhaustion .....................................    8
   10. Reserved AS Numbers ....................................    9
   11. Security Considerations ................................    9
   12. Acknowledgments ........................................    9
   13. References .............................................    9
   14. Authors' Addresses .....................................   10

Hawkinson & Bates        Best Current Practice                  [Page 1]
RFC 1930            Guidelines for creation of an AS          March 1996

1. Introduction

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

2. Motivation

   This memo is aimed at network operators and service providers who
   need to understand under what circumstances they should make use of
   an AS.  It is expected that the reader is familiar with routing
   protocols and will be someone who configures and operates Internet
   networks.  Unfortunately, there is a great deal of confusion in how
   ASes should be used today; this memo attempts to clear up some of
   this confusion, as well as acting as a simple guide to today's
   exterior routing.

3. Definitions

   This document refers to the term "prefix" throughout. In the current
   classless Internet (see [CIDR]), a block of class A, B, or C networks
   may be referred to by merely a prefix and a mask, so long as such a
   block of networks begins and ends on a power-of-two boundary. For
   example, the networks:

        192.168.0.0/24
        192.168.1.0/24
        192.168.2.0/24
        192.168.3.0/24

   can be simply referred to as:

        192.168.0.0/22

   The term "prefix" as it is used here is equivalent to "CIDR block",
   and in simple terms may be thought of as a group of one or more
   networks. We use the term "network" to mean classful network, or "A,
   B, C network".

   The definition of AS has been unclear and ambiguous for some time.
   [BGP-4] states:

Hawkinson & Bates        Best Current Practice                  [Page 2]
RFC 1930            Guidelines for creation of an AS          March 1996

      The classic definition of an Autonomous System is a set of routers
      under a single technical administration, using an interior gateway
      protocol and common metrics to route packets within the AS, and