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. This document recommends specific actions to limit future inconsistency, namely BGP implementors MUST NOT create further inconsistencies from this point forward.

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 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 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.'

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 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 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 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.

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

> When establishing new [RFC1997]-like attributes (large communities, wide communities, etc.), RFC authors should state explicitly how the > new attribute is to be handled.

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.

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

The IANA is requested to list this document as an additional reference for the registry.

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