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rfc:rfc7300

Internet Engineering Task Force (IETF) J. Haas Request for Comments: 7300 Juniper Networks BCP: 6 J. Mitchell Updates: 1930 Microsoft Corporation Category: Best Current Practice July 2014 ISSN: 2070-1721

         Reservation of Last Autonomous System (AS) Numbers

Abstract

 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.  This document updates Section 10 of RFC 1930.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It has been approved for publication by the Internet
 Engineering Steering Group (IESG).  Further information on BCPs is
 available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7300.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Haas & Mitchell Best Current Practice [Page 1] RFC 7300 Last AS Reservation July 2014

1. Introduction

 Over a decade ago, IANA reserved the last Autonomous System Number
 (ASN) of the 16-bit ASN range, 65535, with the intention that it not
 be used by network operators running BGP [RFC4271].  Since the
 introduction of "BGP Support for Four-Octet Autonomous System (AS)
 Number Space" [RFC6793], IANA has also reserved the last ASN of the
 32-bit autonomous system number range, 4294967295.  This reservation
 has been documented in the IANA "Autonomous System (AS) Numbers"
 registry [IANA.AS].  Although these "Last ASNs" border on Private Use
 ASN [RFC6996] ranges, they are not defined or reserved as Private Use
 ASNs by [IANA.AS].  This document describes the reasoning for
 reserving 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

3. Reasons for Reservation of the Last ASNs

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

4. Operational Considerations

 Operators SHOULD NOT use these Last ASNs for any other purpose or as
 Private Use ASNs.  Operational use of these Last ASNs could have
 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 undesirable routing behavior.
 Last ASNs MUST NOT be advertised to the global Internet within
 AS_PATH or AS4_PATH attributes.  Operators SHOULD filter Last ASNs
 within the AS_PATH and AS4_PATH attributes.

Haas & Mitchell Best Current Practice [Page 2] RFC 7300 Last AS Reservation July 2014

5. Implementation Considerations

 While Last ASNs are reserved, they remain valid ASNs from a BGP
 perspective.  Therefore, implementations of BGP [RFC4271] SHOULD NOT
 treat the use of Last ASNs as any type of protocol error.  However,
 if a Last ASN is configured as the local AS, implementations MAY
 generate a warning message indicating 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 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.
 These reservations have been documented in the IANA "Autonomous
 System (AS) Numbers" registry [IANA.AS] and the IANA "Special-Purpose
 Autonomous System (AS) Numbers" registry [IANA.SpecialAS].

7. Security Considerations

 This document does not introduce any additional security concerns in
 regards to usage of Last ASNs.  Although the BGP is designed to allow
 usage of 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",
            <http://www.iana.org/assignments/as-numbers/>.
 [IANA.SpecialAS]
            IANA, "Special-Purpose Autonomous System (AS) Numbers",
            <http://www.iana.org/assignments/
            iana-as-numbers-special-registry/>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.

Haas & Mitchell Best Current Practice [Page 3] RFC 7300 Last AS Reservation July 2014

 [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
            2012.

8.2. Informative References

 [IANA.WK]  IANA, "Border Gateway Protocol (BGP) Well-known
            Communities", <http://www.iana.org/assignments/
            bgp-well-known-communities/>.
 [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
            Communities Attribute", RFC 1997, August 1996.
 [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
            Private Use", BCP 6, RFC 6996, July 2013.

Haas & Mitchell Best Current Practice [Page 4] RFC 7300 Last AS Reservation July 2014

Appendix A. Acknowledgments

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

Authors' Addresses

 Jeffrey Haas
 Juniper Networks
 EMail: jhaas@juniper.net
 Jon Mitchell
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 USA
 EMail: Jon.Mitchell@microsoft.com

Haas & Mitchell Best Current Practice [Page 5]

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