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PROPOSED STANDARD

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
   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.
   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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RFC 8642     Policy Behavior for Well-Known BGP Communities  August 2019


   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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RFC 8642     Policy Behavior for Well-Known BGP Communities  August 2019


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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