Well-Known Community Policy Behavior

The BGP Communities Attribute was specified in which introduced the concept of Well-Known Communities. In hindsight, it 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." One common operational need is to add or remove one or more communities to the current set. Another common need is to replace all received communities with a new set as defined by policy. All BGP policy implementations we know of provide a "set community" directive that operators use to mean "remove any/all communities present on the update received from the neighbor, and apply this set of communities instead."

Vendor implementations differ in the treatment of certain Well-Known communities when modified using the "set community" directive. 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 Juniper Networks' JunOS, "set community" 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 (with a tweezers) 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.

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 share the behavior of their implementations for inclusion in this document, especially if their behavior differs from the examples described. Vendors MUST NOT create additional communities that the "set community" directive would not modify.

Surprising defaults and/or undocumented behaviors are not good for security. This document attepts 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.