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
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. 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
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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.
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
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.
8.1. Normative References
[IANA.AS] IANA, "Autonomous System (AS) Numbers",
IANA, "Special-Purpose Autonomous System (AS) Numbers",
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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.
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