4. COMMUNITIES attribute

4. COMMUNITIES attribute

This document creates the COMMUNITIES path attribute is an optional transitive attribute of variable length. The attribute consists of a set of four octet values, each of which specify a community. All routes with this attribute belong to the communities listed in the attribute.

The COMMUNITIES attribute has Type Code 8.

Communities are treated as 32 bit values, however for administrative assignment, the following presumptions may be made:

The community attribute values ranging from 0x0000000 through 0x0000FFFF and 0xFFFF0000 through 0xFFFFFFFF are hereby reserved.

The rest of the community attribute values shall be encoded using an autonomous system number in the first two octets. The semantics of the final two octets may be defined by the autonomous system (e.g. AS 690 may define research, educational and commercial community values that may be used for policy routing as defined by the operators of that AS using community attribute values 0x02B20000 through 0x02B2FFFF).

Well-known Communities

The following communities have global significance and their operations shall be implemented in any community-attribute-aware BGP speaker.

NO_EXPORT (0xFFFFFF01) : 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).

NO_ADVERTISE (0xFFFFFF02) : All routes received carrying a communities attribute containing this value MUST NOT be advertised to other BGP peers.

NO_EXPORT_SUBCONFED (0xFFFFFF03) : All routes received carrying a communities attribute containing this value MUST NOT be advertised to external BGP peers (this includes peers in other members autonomous systems inside a BGP confederation).