Reservation of Last Autonomous System (AS) Numbers

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IDR                                                              J. Haas
Internet-Draft                                          Juniper Networks
Intended status: Informational                               J. Mitchell
Expires: January 16, 2014                          Microsoft Corporation
                                                           July 15, 2013

           Reservation of Last Autonomous System (AS) Numbers


   This document reserves two Autonomous System numbers (ASNs) at the
   end of the 16 bit and 32 bit ranges, described in this document as
   "Last ASNs" and provides guidance to implementers and operators on
   their use.

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1.  Introduction

   IANA has reserved the last Autonomous System Number (ASN), 65535, of
   the 16 bit autonomous system number range for over a decade with the
   intention that it not be used by network operators running BGP
   [RFC4271].  Since the introduction of BGP Support for Four-Octet AS
   Number Space [RFC6793], IANA has also reserved the ASN of the 32 bit
   autonomous system number range, 4294967295.  This reservation has
   been documented in the IANA Autonomous System Numbers Registry
   [IANA.AS].  Although these "Last ASNs" border on Private Use ASN
   ranges, they are not defined as Private Use ASNs by
   [I-D.ietf-idr-as-private-reservation].  This document describes the
   reasoning for reserving these Last ASNs and provides guidance both to
   operators and to implementers on their use.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Reasons for Last ASNs Reservation

   The primary reason for reserving the Last ASN of both the 16 bit and
   32 bit ASN ranges is that these numbers are also at the end of
   typical computational data structures holding the underlying number.
   Programmatic errors are more common when handling of end of range
   values, and sometimes last values (binary all ones) have been used as
   "magic numbers", to represent a different number or behavior.

   Secondly, a subset of the standard BGP communities of the last ASN of
   the 16 bit range, 65535, are reserved for use by Well-known
   communities as described in [RFC1997] and [IANA.WK].  Although this
   not currently true of ASN 4294967295, if there is a future need for a
   Special Use ASN that is not designed to be globally routable, or the
   associated BGP attributes (such as communities) of such an ASN, this
   could be a valid candidate for such purpose.  This document does not
   prescribe any such purpose to this ASN.

4.  Operational Considerations

   Operators MUST NOT use these Last ASNs as if they are Private Use
   ASNs, and SHOULD NOT use these Last ASNs for any other purpose,
   except a Special Uses defined elsewhere.  Any other operational use
   of these Last ASNs could have unpredictable or undesirable results.
   For example; use of AS 65535 as if it were a Private Use ASN, may
   result in inadvertent use of BGP Well-known community values
   [IANA.WK], causing undesired routing behavior.

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   Operators that choose to filter Private Use ASNs within the AS_PATH
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