Autonomous System (AS) Reservation for Private Use
Reserves a second, much larger block of private AS numbers for networks that do not need globally unique ones.
Mitchell, J., "Autonomous System (AS) Reservation for Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July 2013, <https://www.rfc-editor.org/info/rfc6996>.
@misc{rfc6996,
series = {Request for Comments},
number = 6996,
howpublished = {RFC 6996},
publisher = {RFC Editor},
doi = {10.17487/RFC6996},
url = {https://www.rfc-editor.org/info/rfc6996},
author = {J. Mitchell},
title = {{Autonomous System (AS) Reservation for Private Use}},
pagetotal = 4,
year = 2013,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Mitchell
Request for Comments: 6996 Microsoft Corporation
BCP: 6 July 2013
Updates: 1930
Category: Best Current Practice
ISSN: 2070-1721
Autonomous System (AS) Reservation for Private Use
Abstract
This document describes the reservation of Autonomous System Numbers
(ASNs) that are for Private Use only, known as Private Use ASNs, and
provides operational guidance on their use. This document enlarges
the total space available for Private Use ASNs by documenting the
reservation of a second, larger range and updates RFC 1930 by
replacing Section 10 of that document.
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 represents the consensus of the IETF community. It has
received public review and 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/rfc6996.
Copyright Notice
Copyright (c) 2013 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
The original IANA reservation of Autonomous System Numbers (ASNs) for
Private Use was a block of 1023 ASNs. This was also documented by
the IETF in Section 10 of [RFC1930]. Since the time that the range
was reserved, the Border Gateway Protocol (BGP) [RFC4271] has seen
deployment in new application domains, such as data center networks,
which require a larger Private Use AS space.
Since the introduction of "BGP Support for Four-Octet Autonomous
System (AS) Number Space" [RFC6793], the total size of ASN space has
increased dramatically. A larger subset of the space is available to
network operators to deploy in these Private Use cases. The existing
range of Private Use ASNs is widely deployed, and the ability to
renumber this resource in existing networks cannot be coordinated
given that these ASNs, by definition, are not registered. Therefore,
this RFC documents the existing Private Use ASN reservation while
also introducing a second, larger range that can also be utilized.
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. Private Use ASNs
To allow the continued growth of BGP protocol usage in new network
applications that utilize Private Use ASNs, two ranges of ASNs are
reserved by Section 5 of this document. The first is part of the
original 16-bit Autonomous System range previously defined in
[RFC1930], and the second is a larger range out of the Four-Octet AS
Number Space [RFC6793].
4. Operational Considerations
If Private Use ASNs are used and prefixes originate from these ASNs,
Private Use ASNs MUST be removed from AS path attributes (including
AS4_PATH if utilizing a four-octet AS number space) before being
advertised to the global Internet. Operators SHOULD ensure that all
External Border Gateway Protocol (EBGP) speakers support the
extensions described in [RFC6793] and that implementation-specific
features that recognize Private Use ASNs have been updated to
recognize both ranges prior to making use of the newer, numerically
higher range of Private Use ASNs in the four-octet AS number space.
Some existing implementations that remove Private Use ASNs from the
AS_PATH are known to not remove Private Use ASNs if the AS_PATH
contains a mixture of Private Use and Non-Private Use ASNs. If such
implementations have not been updated to recognize the new range of
ASNs in this document and a mix of old and new range Private Use ASNs
exist in the AS4_PATH, these implementations will likely cease to
remove any Private Use ASNs from either of the AS path attributes.
Normal AS path filtering MAY also be used to prevent prefixes
originating from Private Use ASNs from being advertised to the global
Internet.
5. IANA Considerations
IANA has reserved, for Private Use, a contiguous block of 1023
Autonomous System numbers from the "16-bit Autonomous System Numbers"
registry, namely 64512 - 65534 inclusive.
IANA has also reserved, for Private Use, a contiguous block of
94,967,295 Autonomous System numbers from the "32-bit Autonomous
System Numbers" registry, namely 4200000000 - 4294967294 inclusive.
These reservations have been documented in the IANA "Autonomous
System (AS) Numbers" registry [IANA.AS].
6. Security Considerations
Private Use ASNs do not raise any unique security concerns. Loss of
connectivity might result from their inappropriate use, specifically
outside of a single organization, since they are not globally unique.
This loss of connectivity is limited to the organization using
Private Use ASNs inappropriately or without reference to Section 4.
General BGP security considerations are discussed in [RFC4271] and
[RFC4272]. Identification of the originator of a route with a
Private Use ASN in the AS path would have to be done by tracking the
route back to the neighboring globally unique AS in the path or by
inspecting other attributes.
7. References
7.1. 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.
7.2. Informative References
[IANA.AS] IANA, "Autonomous System (AS) Numbers",
<http://www.iana.org/assignments/as-numbers/>.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, March 1996.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, January 2006.
8. Acknowledgements
The author would like to acknowledge Christopher Morrow, Jason
Schiller, and John Scudder for their advice on how to pursue this
change. The author would also like to thank Brian Dickson, David
Farmer, Jeffrey Haas, Nick Hilliard, Joel Jaeggli, Warren Kumari, and
Jeff Wheeler for their comments and suggestions.
Author's Address
Jon Mitchell
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
EMail: Jon.Mitchell@microsoft.com
AI ELI5
On this page
§1Why a Bigger Private Range
The original IANA reservation of Private Use Autonomous System Numbers (ASNs) was a block of 1023 numbers, also documented in RFC 1930. Since then, BGP has been deployed in new domains such as data center networks, which need a larger private space.
The four-octet AS space from RFC 6793 made the total ASN space much bigger. The existing private range is widely deployed and cannot be renumbered, because these ASNs are by definition not registered. So the RFC documents the existing reservation and adds a second, larger range.
The neighborhood ran out of house numbers and could not rename its streets, so a new, huge neighborhood was added next to it.
§2The Two Private Use Ranges
IANA reserved a contiguous block of 1023 numbers from the 16-bit registry, 64512 - 65534 inclusive. It also reserved a second contiguous block from the 32-bit registry, 4200000000 - 4294967294 inclusive. The first range is part of the original 16-bit space from RFC 1930, and the second is part of the four-octet space.
There are now two private ranges: the small old one and a large new one.
§3Operational Rules
If prefixes originate from Private Use ASNs, those ASNs MUST be removed from AS path attributes (including AS4_PATH) before being advertised to the global Internet. Normal AS path filtering MAY also be used.
Operators SHOULD make sure all EBGP speakers support RFC 6793 and that features recognizing Private Use ASNs know both ranges before using the new range. Some implementations do not remove private ASNs when the path mixes private and non-private ones. If they do not know the new range, they will likely stop removing private ASNs from either path attribute.
Private numbers must be taken off the label before a letter leaves your neighborhood, and your mail sorters must know about the new neighborhood first.
§4Security Considerations
Private Use ASNs raise no unique security concerns. Loss of connectivity might result from using them outside a single organization, since they are not globally unique, but that loss is limited to the organization doing so. Finding the originator of a route with a private ASN means tracking it back to the neighboring globally unique AS or inspecting other attributes.
If you use a neighborhood-only number outside the neighborhood, only you get confused.