Network Working Group                                     J. Borkenhagen
Internet-Draft                                                      AT&T
Updates: 1997 (if approved)                                      R. Bush
Intended status: Standards Track               Internet Initiative Japan
Expires: December 1, 12, 2019                                     R. Bonica
                                                        Juniper Networks
                                                            S. Bayraktar
                                                           Cisco Systems
                                                            May 30,
                                                           June 10, 2019

                  Well-Known Community Policy Behavior


   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

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   be interpreted as described in RFC 2119 [RFC2119] only when they
   appear in all upper case.  They may also appear in lower or mixed
   case as English words, without normative meaning.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 1, 12, 2019.

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Table of Contents

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

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.

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 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
   syntax to "set" community that operators use to mean "remove any/all
   communities present on the route, and apply this set of communities

   Some operators prefer to write explicit policy to delete unwanted
   communities rather than using "set;" i.e. using a "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 both
   "set" and "delete community *:*" syntax.  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, while 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.

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) |

                  Communities not removed by Cisco IOS XR

                                  Table 1

   Cisco IOS XR does allow Well-Known communities to be removed only by
   explicitly enumerating one at a time, 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

   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

4.1.  Note on an Inconsistency

   The 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
   specific community on Cisco IOS XR's special treatment list (internet
   == 0:0) is not really on IANA's list -- 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."

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

   > When establishing new [RFC1997]-like attributes (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 SHOULD clearly document the behavior of "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.  I.e.,
   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.

7.  Security Considerations

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

8.  IANA Considerations

   This document has no IANA Considerations; though the

   The IANA may wish to
   refer is requested to list this document, if/when published, in [IANA-WKC]. document as an additional
   reference for the [IANA-WKC] registry.

9.  Acknowledgments

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

10.  Normative References

              IANA, "Border Gateway Protocol (BGP) Well-Known
              Communities", <

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

Authors' Addresses

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


   Randy Bush
   Internet Initiative Japan
   5147 Crystal Springs
   Bainbridge Island, WA  98110
   United States of America

   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, VA  20171


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