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Internet Engineering Task Force (IETF)                    J. BorkenhagenRequest for Comments: 8642                                          AT&TUpdates:1997                                                    R. BushCategory: Standards Track                                   IIJ & ArrcusISSN: 2070-1721                                                R. Bonica                                                        Juniper Networks                                                            S. Bayraktar                                                           Cisco Systems                                                             August 2019Policy Behavior for Well-Known BGP CommunitiesAbstract   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   updateRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8642.Borkenhagen, et al.          Standards Track                    [Page 1]

RFC 8642     Policy Behavior for Well-Known BGP Communities  August 2019Copyright Notice   Copyright (c) 2019 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Manipulation of Communities by Policy . . . . . . . . . . . .33.  Community Manipulation Policy Differences . . . . . . . . . .34.  Documentation of Vendor Implementations . . . . . . . . . . .44.1.  Note on an Inconsistency  . . . . . . . . . . . . . . . .5   5.  Note for Those Writing RFCs for New Community-Like Attributes   56.  Action Items  . . . . . . . . . . . . . . . . . . . . . . . .57.  Security Considerations . . . . . . . . . . . . . . . . . . .68.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .69.  Normative References  . . . . . . . . . . . . . . . . . . . .6   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .71.  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.Borkenhagen, et al.          Standards Track                    [Page 2]

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 inBCP 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.Borkenhagen, et al.          Standards Track                    [Page 3]

RFC 8642     Policy Behavior for Well-Known BGP Communities  August 20194.  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-ownrfc7611                |            | 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.Borkenhagen, et al.          Standards Track                    [Page 4]

RFC 8642     Policy Behavior for Well-Known BGP Communities  August 20194.1.  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.Borkenhagen, et al.          Standards Track                    [Page 5]

RFC 8642     Policy Behavior for Well-Known BGP Communities  August 20197.  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 inRFC2119 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.Borkenhagen, et al.          Standards Track                    [Page 6]

RFC 8642     Policy Behavior for Well-Known BGP Communities  August 2019Authors' 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.comBorkenhagen, et al.          Standards Track                    [Page 7]