Network Working Group                                     J. Borkenhagen
Internet-Draft                                                      AT&T
Intended status: Standards Track                                 R. Bush
Expires: September 11, 2019                    Internet Initiative Japan
                                                               R. Bonica
                                                        Juniper Networks
                                                            S. Bayraktar
                                                           Cisco Systems
                                                          March 10, 2019

                  Well-Known Community Policy Behavior


   Well-Known BGP Communities are manipulated inconsistently by current
   implementations.  This results in difficulties for operators.
   Network operators are encouraged to deploy consistent community
   handling across their networks, taking the inconsistent behaviors
   from the various BGP implementations they operate into consideration.
   Also, BGP implementors are expected to not create any further
   inconsistencies from this point forward.

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 11, 2019.

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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
   ( 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 . . . . . . . . . . .   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.

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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 received 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 instead."

   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 received 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.

   In Juniper Networks' Junos OS, "community set" removes all received
   communities, Well-Known or otherwise.

   In Cisco Systems' IOS XR, "set community" removes all received
   communities except for the following:

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            | 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

   IOS XR does allow Well-Known communities to be removed one at a time
   by explicit policy; for example, "delete community accept-own".
   Operators are advised to consult IOS XR documentation and/or Cisco
   Systems 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 received
   communities, well-Known or otherwise.

   In OpenBSD's 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-WKS].

   IOS XR's set of well-known communities that "set community" will not
   overwrite diverges from IANA's list.  Quite a few well-known
   communities from IANA's list do not receive special treatment in IOS
   XR, and at least one specific community on 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."

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

   Care should be taken when establishing new [RFC1997]-like attributes
   (large communities, wide communities, etc) to avoid repeating this

6.  Action Items

   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.

   Network operators are encouraged to limit their use of the "set"
   directive (within reason), to improve the readability of their
   configurations and hopefully to achieve behavioral consistency across

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 other than to be aware that
   any future Well-Known Communities will be subject to the policy
   treatment described here.

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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 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


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   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


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