Reservation of Last Autonomous System (AS) Numbers
Explains why the last AS numbers of the 16-bit and 32-bit ranges are reserved and should not be used.
Haas, J. and J. Mitchell, "Reservation of Last Autonomous System (AS) Numbers", BCP 6, RFC 7300, DOI 10.17487/RFC7300, July 2014, <https://www.rfc-editor.org/info/rfc7300>.
@misc{rfc7300,
series = {Request for Comments},
number = 7300,
howpublished = {RFC 7300},
publisher = {RFC Editor},
doi = {10.17487/RFC7300},
url = {https://www.rfc-editor.org/info/rfc7300},
author = {J. Haas and J. Mitchell},
title = {{Reservation of Last Autonomous System (AS) Numbers}},
pagetotal = 5,
year = 2014,
month = jul,
} RFC text
Contents
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.
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.
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.
[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.
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
AI ELI5
On this page
§1Introduction
IANA reserved the last ASN of the 16-bit range, 65535, over a decade before this RFC. Since RFC 6793, IANA has also reserved the last ASN of the 32-bit range, 4294967295. Although these Last ASNs border on the Private Use ranges of RFC 6996, they are not reserved as Private Use ASNs. The RFC gives the reasoning and guidance for operators and implementers.
The last number on each list is set aside, and it is not one of the free-to-use private numbers.
§2Reasons for Reservation
Some BGP communities of ASN 65535 are reserved for Well-known Communities, as described in RFC 1997. This is not currently true of 4294967295, but if a future Special Use ASN is needed that is not designed to be globally routable, that number could be a candidate. The document prescribes no such use at publication.
One last number is already used for special messages, and the other is kept free in case a special job turns up.
§3Guidance for Operators
Operators SHOULD NOT use Last ASNs for any other purpose or as Private Use ASNs. For example, using AS 65535 as private might accidentally trigger BGP Well-known Community values and cause undesirable routing. Last ASNs MUST NOT be advertised to the global Internet within AS_PATH or AS4_PATH, and operators SHOULD filter them there.
Do not use the reserved last number as your own, and do not let it leak out to the wider world.
§4Guidance for Implementers
Last ASNs remain valid ASNs from a BGP perspective, so implementations SHOULD NOT treat their use as a protocol error. If a Last ASN is configured as the local AS, an implementation MAY warn about improper use of a reserved ASN. Tools that filter Private Use ASNs MAY also include Last ASNs.
On security, the RFC adds no new concerns, though BGP implementation errors could be triggered by Last ASN usage.
Routers should not panic if they see the number, though they may politely warn.