IDR J. Haas
Internet-Draft Juniper Networks
Intended status: Informational J. Mitchell
Expires: October 10, 2014 Microsoft Corporation
April 08, 2014

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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on October 10, 2014.

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 ( 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.

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 last 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 [RFC6996] ranges, they are not defined or reserved as Private Use ASNs by [IANA.AS]. 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

3. Reasons for Last ASNs Reservation

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 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 as if they are Private Use ASNs or for any other purpose. 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 undesireable routing behavior.

These last ASNs MUST NOT be advertised to the global Internet within AS_PATH or AS4_PATH attributes. Operators that choose to do filtering of Private Use ASNs within the AS_PATH and AS4_PATH attributes SHOULD also filter these Last ASNs.

5. Implementation Considerations

While these Last ASNs are reserved, they remain valid ASNs from a BGP 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 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

[Note to IANA, TO BE REMOVED BEFORE PUBLICATION: IANA please update the reservations for values 65535 and 4294967295 in the three registries mentioned below to reference this document. Please note at the time of this comment the "Special-Purpose AS Numbers registry" has not yet been created but should be before publication of this document.]

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].

This reservation has also been documented in the IANA Special-Purpose AS Numbers Registry as described by [I-D.housley-number-registries].

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

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[IANA.AS] IANA, , "Autonomous System (AS) Numbers", April 2014.
[I-D.housley-number-registries] Housley, R., "Internet Numbers Registries", Internet-Draft draft-housley-number-registries-04, February 2014.

8.2. Informative References

[IANA.WK] IANA, , "Border Gateway Protocol (BGP) Well-known Communities", April 2014.
[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.

Appendix A. Acknowledgments

The authors would like to thank Michelle Cotton and Elwyn Davis 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:
Jon Mitchell Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA EMail:

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