Well-Known Community Policy Behavior

The BGP Communities Attribute was specified in which introduced the concept of Well-Known Communities. In hindsight, 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.

says: "A BGP speaker receiving a route with the COMMUNITIES path attribute may modify this attribute according to the local policy." A basic operational need is to add or remove one or more communities to the received set. Another common need is to replace all received communities with a new set. To simplify the second case, most BGP policy implementations provide syntax to "set" community that operators use to mean "remove any/all communities present on the update received from the neighbor, 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.

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.

In this section we document the syntax and observed behavior of the "set" directive in several popular BGP implementations. In Juniper Networks' JunOS, "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: 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 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 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.

The IANA publishes a list of Well-Known Communities . 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."

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

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 any Well-Known Communities specified after this document's publication are removed by their "set" directive. 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 platforms.

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

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.

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