BGP community

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A BGP community is an attribute, attached to an announcement of a route to which the sender offers connectivity. Communities are most often identifiers for groups of routes/addresses to which some common policy applies. [1] There are both well-known communities that should be recognized by all BGP implementations, and various kinds of communities that are usually defined by an autonomous system [2]

To deal with Internet growth and the use of BGP in intranets and extranets (e.g., virtual private networks), various extended communities have been defined. [3] These primarily deal with internet operations issues, such as the scope of routing information distribution.

Basic structure of a community identifier

As first defined, a community is a 32-bit binary string, broken into two 16-bit fields. The first field's value is either all binary ones, indicating it is a "well-known" community, or contains the value of the autonomous system that defines the meaning of the second field.

By convention, a community is written:

ASN or 65535:specific meaning

Many router implementations will allow the ASN field to be displayed in decimal rather than hexadecimal, corresponding to general practice in BGP routing.

Well-known communities

A BGP implementation supporting communities MUST understand the following well-known communities

Representative non-national TLD registries
Name Meaning Value
NO-EXPORT Do not advertise this route outside the current autonomous system FFFF:1
NO-ADVERTISE Do not advertise this route to any other BGP router, even inside the same AS. FFFF:2
NO_EXPORT_SUBCONFED All routes received carrying a communities attribute containing this value MUST NOT be advertised outside a BGP confederation boundary (a stand-alone autonomous system that is not part of a confederation should be considered a confederation itself). FFFF:3

Since the ASNs from decimal 64512 through 65535 are reserved, these effectively are NO-EXPORT onto the Internet.

References

  1. Chandra R., Traina P., Li T. (August 1996), BGP Communities Attribute, RFC1997
  2. There are communities, typically used in intranets and extranets, where a prefix other than an autonomous system number is used to disambiguate
  3. Tappan D., Rekhter Y., Sangli I. (February 2006), BGP Extended Communities Attribute, RFC4360