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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   This Internet-Draft will expire on January 16, 2014.

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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
   and AS4_PATH attributes SHOULD also filter these Last ASNs.  These
   last ASNs MUST NOT be advertised to the global Internet within
   AS_PATH or AS4_PATH attributes.

5.  Implementation Considerations

   While these Last ASNs are reserved, they remain valid ASNs from a
   protocol perspective.  Therefore, implementations of BGP [RFC4271]
   SHOULD NOT treat the use of these Last ASNs as any type of protocol
   error.  However, implementations MAY generate a local warning message
   indicating probable improper use of a reserved ASN.

   Implementations that provide tools that filter Private Use ASNs
   within the AS_PATH and AS4_PATH attributes MAY also include these
   Last ASNs.

6.  IANA Considerations

   IANA has reserved last Autonomous System number 65535 from the
   "16-bit Autonomous System Numbers" registry for the reasons described
   in this document.

   IANA has also reserved last Autonomous System number 4294967295 from
   the "32-bit Autonomous System Numbers" registry for the reasons
   described in this document.

   This reservation has been documented in the IANA Autonomous System
   Numbers Registry [IANA.AS].

7.  Security Considerations

   This document does not introduce any additional security concerns in
   regards to usage of Last ASNs.  Although the BGP protocol is designed
   to allow usage of these Last ASNs, security issues related to BGP
   implementation errors could be triggered by Last ASN usage.

8.  References

8.1.  Normative References

   [IANA.AS]  IANA, ., "Autonomous System (AS) Numbers", July 2013,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December

8.2.  Informative References

              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-ietf-idr-as-private-reservation-05
              (work in progress), May 2013.

   [IANA.WK]  IANA, ., "Border Gateway Protocol (BGP) Well-known
              Communities", July 2013, <

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

Appendix A.  Acknowledgements

   The authors would like to thank Michelle Cotton and Elwyn Davis for
   encouraging the proper documentation of the reservation of these
   ASNs.  The authors would also like to thank David Farmer for his
   contributions to the document.

Authors' Addresses

   Jeffrey Haas
   Juniper Networks


   Jon Mitchell
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


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