RFC 7300

Reservation of Last Autonomous System (AS) Numbers

📅 published July 2014, from ietf

Reservation of Last Autonomous System (AS) Numbers 💖

Quick facts:

  • RFC: 7300
  • BCP: 6
  • Category: Best Current Practice
  • Date: July 2014
  • ISSN: 2070-1721
  • Updates: RFC 1930 (Section 10 of it)
  • Obsoletes: nothing! 🙅‍♀️
  • Authors: J. Haas (Juniper Networks) and J. Mitchell (Microsoft Corporation)
  • Publisher: Internet Engineering Task Force (IETF) 🎀

Abstract

okay so this doc reserves two ASNs at the very end of the 16-bit and 32-bit ranges. it calls them "Last ASNs" 🦋 and gives implementers and operators the tea on how to handle them. it also updates Section 10 of RFC 1930.

Status of This Memo

this is a Best Current Practice (a BCP) from the Internet Community 💅

it's an IETF product, and the IESG (Internet Engineering Steering Group) approved it for publication. if you want more BCP info, check Section 2 of RFC 5741.

for the current status, errata, or to send feedback, go to http://www.rfc-editor.org/info/rfc7300 🧚

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. 🌈

this doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as of the publication date. please read these carefully, bestie, because they explain what you can and can't do with it. 📜

if you pull Code Components out of this doc, they have to include the Simplified BSD License text (as described in Section 4.e of the Trust Legal Provisions), and they come with no warranty as described in that license. 🔥

1. Introduction ✨

okay the backstory is kinda iconic. over a decade ago, IANA reserved the last ASN in the 16-bit range, 65535, so network operators running BGP [RFC4271] wouldn't use it. then, once [RFC6793] came out with "BGP Support for Four-Octet Autonomous System (AS) Number Space", IANA reserved the last ASN in the 32-bit range too, 4294967295. 😭

that reservation is written down in the IANA "Autonomous System (AS) Numbers" registry [IANA.AS]. these "Last ASNs" are kinda close to the Private Use ASN [RFC6996] ranges, but [IANA.AS] doesn't define or reserve them as Private Use ASNs, so they're their own thing. 🙈

this doc explains why the Last ASNs got reserved and gives guidance to both operators and implementers on using them. main character energy 👑

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]. this is the rule book for all the caps-lock words in here 💜

3. Reasons for Reservation of the Last ASNs 🦋

so, a bunch of BGP communities under ASN 65535 (the last one in the 16-bit range) are set aside for Well-known Communities, as described in [RFC1997] and [IANA.WK]. 🌸

it's not like that for ASN 4294967295 right now. BUT if someday there's a need for another Special Use ASN that isn't meant to be globally routable (or for its BGP communities), 4294967295 could be a solid candidate. this doc doesn't assign any special use to it at publication time, though. ✨

4. Operational Considerations 💅

the big rule: operators SHOULD NOT use these Last ASNs for any other purpose or as Private Use ASNs. using them could cause some seriously undesirable results. 💀

like, if you treat AS 65535 as if it's a Private Use ASN, you might accidentally use BGP Well-known Community values [IANA.WK], and that can cause undesirable routing behavior. not a vibe 😭

and the other big rule is that Last ASNs MUST NOT be advertised to the global Internet in AS_PATH or AS4_PATH attributes. operators SHOULD filter Last ASNs out of the AS_PATH and AS4_PATH attributes. 🔥

Last ASNs must never leak into the global AS_PATH or AS4_PATH. filter them out, bestie. 🚫

5. Implementation Considerations 🧚

okay so even though Last ASNs are reserved, they're still valid ASNs from a BGP point of view. 🫶 that means implementations of BGP [RFC4271] SHOULD NOT treat using a Last ASN as any kind of protocol error.

BUT! if a Last ASN gets configured as the local AS, implementations MAY generate a warning message saying it's an improper use of a reserved ASN. 🙈

also, tools that filter Private Use ASNs out of the AS_PATH and AS4_PATH attributes MAY include Last ASNs in that filtering too. ✨

6. IANA Considerations 🌈

IANA has reserved the last Autonomous System number, 65535, from the "16-bit Autonomous System Numbers" registry for the reasons explained in this doc. 🎀

IANA has also reserved the last Autonomous System number, 4294967295, from the "32-bit Autonomous System Numbers" registry, for the same reasons. 💖

both reservations are 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 doc doesn't add any new security concerns when it comes to using Last ASNs. 🙂 BUT, and this is a big but, BGP is designed to allow Last ASN usage, and security issues from BGP implementation errors could get triggered by using them. so watch out for those bugs, okay?? 😭💀

8. References 💜

8.1. Normative References

8.2. Informative References

Appendix A. Acknowledgments 🫶

the authors want to thank Michelle Cotton and Elwyn Davies for encouraging the proper documentation of these ASN reservations, and David Farmer for his contributions to the doc. the real MVPs 🏆

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

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