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BGP Communities Attribute

BGP communities attribute is widely used for implementing policy routing. Network operators can manipulate BGP communities attribute based on their network policy. BGP communities attribute is defined in RFC1997 - BGP Communities Attribute and RFC1998 - An Application of the BGP Community Attribute in Multi-home Routing. It is an optional transitive attribute, therefore local policy can travel through different autonomous system.

Communities attribute is a set of communities values. Each communities value is 4 octet long. The following format is used to define communities value.

AS:VAL
This format represents 4 octet communities value. AS is high order 2 octet in digit format. VAL is low order 2 octet in digit format. This format is useful to define AS oriented policy value. For example, 7675:80 can be used when AS 7675 wants to pass local policy value 80 to neighboring peer.
internet
internet represents well-known communities value 0.
no-export
no-export represents well-known communities value NO_EXPORT
(0xFFFFFF01). All routes carry this value must not be advertised to outside a BGP confederation boundary. If neighboring BGP peer is part of BGP confederation, the peer is considered as inside a BGP confederation boundary, so the route will be announced to the peer.
no-advertise
no-advertise represents well-known communities value NO_ADVERTISE
(0xFFFFFF02). All routes carry this value must not be advertise to other BGP peers.
local-AS
local-AS represents well-known communities value NO_EXPORT_SUBCONFED (0xFFFFFF03). All routes carry this value must not be advertised to external BGP peers. Even if the neighboring router is part of confederation, it is considered as external BGP peer, so the route will not be announced to the peer.

When BGP communities attribute is received, duplicated communities value in the communities attribute is ignored and each communities values are sorted in numerical order.