BGP community

A BGP community is an attribute, attached to an announcement of a route to which the sender offers connectivty. Communities are most often identifiers for groups of routes/addresses to which some common policy applies. 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

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.

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:

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

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