RFC 8642

Policy Behavior for Well-Known BGP Communities

📅 published August 2019, from ietf

RFC 8642: Policy Behavior for Well-Known BGP Communities 🎀

Okay bestie, this is RFC 8642, and it's giving "official Internet standard" 💅 It came out in August 2019 as a Standards Track document, which means it's a real spec and not just someone's opinion. The authors are J. Borkenhagen (AT&T), R. Bush (IIJ & Arrcus), R. Bonica (Juniper Networks), and S. Bayraktar (Cisco Systems). It updates RFC 1997 and doesn't obsolete anything. The ISSN is 2070-1721, and it's an IETF product ✨

Abstract

Well-known BGP communities get manipulated differently across current implementations, and that's a whole mess for operators 😭 Network operators should deploy consistent community handling across their networks while taking the inconsistent behaviors from the various BGP implementations into consideration. This doc recommends specific actions to limit future inconsistency: basically, BGP implementors must not create further inconsistencies from this point forward. These behavioral changes are subtle, but they actually update RFC 1997 🦋

Status of This Memo

This is an Internet Standards Track document.

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 Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8642.

Copyright (c) 2019 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 (https://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.

Table of Contents

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
2.  Manipulation of Communities by Policy . . . . . . . . . . . .   3
3.  Community Manipulation Policy Differences . . . . . . . . . .   3
4.  Documentation of Vendor Implementations . . . . . . . . . . .   4
  4.1.  Note on an Inconsistency  . . . . . . . . . . . . . . . .   5
5.  Note for Those Writing RFCs for New Community-Like Attributes   5
6.  Action Items  . . . . . . . . . . . . . . . . . . . . . . . .   5
7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
9.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1. Introduction ✨

The BGP Communities attribute was specified in [RFC1997], which introduced the concept of well-known communities. In hindsight, [RFC1997] did not prescribe as fully as it should have how well-known communities may be manipulated by policies applied by operators. Right now implementations differ on this, and those differences can lead to inconsistent behaviors that operators find really hard to spot and fix 😩

This document describes the current behavioral differences so operators can write consistent community-manipulation policies in a multi-vendor environment, and so we don't add even more divergence in implementations 🌸

This document recommends specific actions to limit future inconsistency: namely, BGP implementors MUST NOT create further inconsistencies from this point forward. 🔥

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Manipulation of Communities by Policy 🧚

[RFC1997] says:

A BGP speaker receiving a route with the COMMUNITIES path attribute may modify this attribute according to the local policy.

One basic operational need is adding or removing one or more communities to or from the set. The focus of this document is another really common need: replacing all communities with a new set. To keep that second case simple, most BGP policy implementations give you a syntax to "set" a community, which operators use to mean "remove any/all communities present on the route and apply this set of communities instead" 💜

Some operators would rather write explicit policy that deletes unwanted communities, instead of using "set". So they use delete community *:* and then add community x:y ... configuration statements to try to replace everything. The same community-manipulation policy differences described in the next section exist in the syntax for both "set" and delete community *:*. To keep things simple, the rest of this document only talks about the "set" behaviors, which we collectively call each implementation's '"set" directive' 🎀

3. Community Manipulation Policy Differences 🦋

Vendor implementations differ in how they treat certain well-known communities when they're modified using the syntax to "set" the community. Some replace all communities, including the well-known ones, with the new set. Others replace all non-well-known communities but don't modify any well-known communities that are already present 👀

These differences mean that what looks like identical policy configurations can have very different results on different platforms. Iconic chaos, honestly 💀

4. Documentation of Vendor Implementations 💅

In this section, we document the syntax and observed behavior of the "set" directive in several popular BGP implementations, to show how bad the problem is for operators 😭

In Juniper Networks' Junos OS, "community set" removes all communities, well-known or otherwise.

In Cisco IOS XR, "set community" removes all communities except for the ones in the table below 🫶

+-------------+-----------------------------------+
| Numeric     | Common Name                       |
+-------------+-----------------------------------+
| 0:0         | internet                          |
| 65535:0     | graceful-shutdown                 |
| 65535:1     | accept-own rfc7611                |
| 65535:65281 | NO_EXPORT                         |
| 65535:65282 | NO_ADVERTISE                      |
| 65535:65283 | NO_EXPORT_SUBCONFED (or local-AS) |
+-------------+-----------------------------------+

Table 1: Communities Not Removed by Cisco's IOS XR

Cisco IOS XR only lets you remove well-known communities by listing them explicitly one at a time, not in the aggregate. For example, delete community accept-own. Operators are advised to check Cisco IOS XR documentation and/or Cisco support for the full details 📚

On Extreme networks' Brocade NetIron, "set community X" removes all communities and sets X 🍓

In Huawei's VRP product, "community set" removes all communities, well-known or otherwise.

In OpenBGPD, "set community" does not remove any communities, well-known or otherwise. 🌈

Nokia's SR OS has several directives that operate on communities. Its "set" directive is called using the "replace" keyword, which replaces all communities, well-known or otherwise, with the specified communities ✨

4.1. Note on an Inconsistency 🧁

IANA publishes a list of well-known communities [IANA-WKC].

Cisco IOS XR's set of well-known communities that "set community" will not overwrite diverges from IANA's list of well-known communities. Quite a few well-known communities from IANA's list don't get special treatment in Cisco IOS XR. And at least one community on Cisco IOS XR's special treatment list, internet == 0:0, isn't formally a well-known community, since it isn't in [IANA-WKC] (it's taken from the Reserved range [0x00000000-0x0000FFFF]) 🙈

This merely notes an inconsistency. It is not a plea to protect the entire IANA list from "set community".

Authors' Addresses 💌

Okay bestie, here are the four amazing people who wrote this one, with their addresses and emails so you can slide into their DMs 💖

Jay Borkenhagen 🌸

Jay Borkenhagen
AT&T
200 Laurel Avenue South
Middletown, NJ  07748
United States of America

Email: jayb@att.com

Randy Bush 🦋

Randy Bush
IIJ & Arrcus
5147 Crystal Springs
Bainbridge Island, WA  98110
United States of America

Email: randy@psg.com

Ron Bonica 🎀

Ron Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA  20171
United States of America

Email: rbonica@juniper.net

Serpil Bayraktar ✨

Serpil Bayraktar
Cisco Systems
170 W. Tasman Drive
San Jose, CA  95134
United States of America

Email: serpil@cisco.com

That's the whole squad, no cap 👑 🫶

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