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.
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 represents well-known communities value 0.
no-export represents well-known communities value
(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 represents well-known communities value
(0xFFFFFF02). All routes carry this value must not be advertise to other BGP peers.
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.
|• BGP Community Lists:|
|• Numbered BGP Community Lists:|
|• BGP Community in Route Map:|
|• Display BGP Routes by Community:|
|• Using BGP Communities Attribute:|