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Internet Engineering Task Force (IETF)                           J. HaasRequest for Comments: 7300                              Juniper NetworksBCP: 6                                                       J. MitchellUpdates:1930                                      Microsoft CorporationCategory: Best Current Practice                                July 2014ISSN: 2070-1721Reservation of Last Autonomous System (AS) NumbersAbstract   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 updatesSection 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 20141.  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 inRFC 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 20145.  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.  References8.1.  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 2014Appendix 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.comHaas & Mitchell           Best Current Practice                 [Page 5]