    Internet Engineering Task Force (IETF)                    J. Borkenhagen
    Request for Comments: 8642                                          AT&T
    Updates: 1997                                                    R. Bush
    Category: Standards Track                                   IIJ & Arrcus
    ISSN: 2070-1721                                                R. Bonica
                                                            Juniper Networks
                                                                S. Bayraktar
                                                               Cisco Systems
                                                                 August 2019

                 Policy Behavior for Well-Known BGP Communities

    Abstract

       Well-known BGP communities are manipulated differently across various
       current implementations, resulting in difficulties 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 document
       recommends specific actions to limit future inconsistency: namely,
       BGP implementors must not create further inconsistencies from this
       point forward.  These behavioral changes, though subtle, 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.

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    Copyright Notice

       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
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       (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
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       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.
       Currently, implementations differ in this regard, and these
       differences can result in inconsistent behaviors that operators find
       difficult to identify and resolve.

       This document describes the current behavioral differences in order
       to assist operators in generating consistent community-manipulation
       policies in a multi-vendor environment and to prevent the
       introduction of additional divergence in implementations.

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

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       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 to add or remove one or more
       communities to or from the set.  The focus of this document is
       another common operational need: to replace all communities with a
       new set.  To simplify this second case, most BGP policy
       implementations provide a syntax to "set" a community that operators
       use to mean "remove any/all communities present on the route and
       apply this set of communities instead".

       Some operators prefer to write explicit policy to delete unwanted
       communities rather than use "set", i.e., using "delete community *:*"
       and then "add community x:y ..." configuration statements in an
       attempt to replace all communities.  The same community-manipulation
       policy differences described in the following section exist in the
       syntax for both "set" and "delete community *:*".  For simplicity,
       the remainder of this document refers only to the "set" behaviors,
       which we refer to collectively as each implementation's '"set"
       directive'.

    3.  Community Manipulation Policy Differences

       Vendor implementations differ in the treatment of certain well-known
       communities when 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 do not
       modify any well-known communities that are present.

       These differences result in what would appear to be identical policy
       configurations having very different results on different platforms.

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    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 illustrate
       the severity of the problem operators face.

       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 following:

                +-------------+-----------------------------------+
                | 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 allows well-known communities to be removed only by
       explicitly enumerating one at a time and not in the aggregate -- for
       example, "delete community accept-own".  Operators are advised to
       consult Cisco IOS XR documentation and/or Cisco support for 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, replacing
       all communities, well-known or otherwise, with the specified
       communities.

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    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 the IANA's list of well-known
       communities.  Quite a few well-known communities from IANA's list do
       not receive special treatment in Cisco IOS XR, and at least one
       community on Cisco IOS XR's special treatment list, internet == 0:0,
       is not formally a well-known community as it is not in [IANA-WKC] (it
       is 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".

    5.  Note for Those Writing RFCs for New Community-Like Attributes

       When establishing new attributes similar to those in [RFC1997] (large
       communities, wide communities, etc.), RFC authors should state
       explicitly how the new attribute is to be handled.

    6.  Action Items

       Network operators are encouraged to limit their use of the "set"
       directive (within reason) to improve consistency across platforms.

       Unfortunately, it would be operationally disruptive for vendors to
       change their current implementations.

       Vendors MUST clearly document the behavior of the "set" directive in
       their implementations.

       Vendors MUST ensure that their implementations' "set" directive
       treatment of any specific community does not change if/when that
       community becomes a new well-known community through future
       standardization.  For most implementations, this means that the "set"
       directive MUST continue to remove the community; for those
       implementations where the "set" directive removes no communities,
       that behavior MUST continue.

       Given the implementation inconsistencies described in this document,
       network operators are urged never to rely on any implicit
       understanding of a neighbor ASN's BGP community handling.  That is,
       before announcing prefixes with NO_EXPORT or any other community to a
       neighbor ASN, the operator should confirm with that neighbor how the
       community will be treated.

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    7.  Security Considerations

       Surprising defaults and/or undocumented behaviors are not good for
       security.  This document attempts to remedy that.

    8.  IANA Considerations

       The IANA has listed this document as an additional reference for the
       [IANA-WKC] registry.

    9.  Normative References

       [IANA-WKC] IANA, "Border Gateway Protocol (BGP) Well-known
                  Communities", <https://www.iana.org/assignments/
                  bgp-well-known-communities>.

       [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
                  Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
                  <https://www.rfc-editor.org/info/rfc1997>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

    Acknowledgments

       The authors thank Martijn Schmidt and Qin Wu for the Huawei data
       point as well as Greg Hankins, Job Snijders, David Farmer, John
       Heasley, and Jakob Heitz.

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    Authors' Addresses

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

       Email: jayb@att.com

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

       Email: randy@psg.com

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

       Email: rbonica@juniper.net

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

       Email: serpil@cisco.com

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