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Chapter 9
BGP Scalability and Advanced Features
Lastly, let's look at RouterC's configuration, which is very similar to that
of RouterB.
RouterC(config)#router bgp 3
RouterC(config-router)#bgp confederation identifier 31400
RouterC(config-router)#bgp confederation peers 1 2
RouterC(config-router)#neighbor remote-as 1
Peer Groups and Communities
Peer groups and communities are used to eliminate some of the overhead
associated with BGP. BGP, as you can see, is a very complex protocol with
many configuration options. Communities are a way of tagging routes to
make sure that a consistent filtering or route-selection policy exists when
using route maps.
All the BGP routers can tag routes coming into or going out of their inter-
faces when doing routing updates. The COMMUNITIES attribute (Type
Code 8) is used to carry the communities information in the BGP update
packets. BGP routers can then filter routes in incoming or outgoing updates
or use preferred routes based on the COMMUNITIES attribute. By default,
the communities information is stripped from any outgoing BGP update.
Without any communities configured, each individual BGP neighbor would
require either a statement in an access list for a distribute list or a statement
in a prefix list.
Some implementations do not understand the concept of communities.
When such is the case, the router will still send the information onto the next
router. When the implementation does understand the concept of commu-
nities, then the router must be configured to propagate the COMMUNITIES
attribute--otherwise, the communities information will be dropped.
The COMMUNITIES attribute can contain a value in the range of 0 to
4,294,967,200. Remember that a network can be a member of multiple
communities and that you can use route maps to set the community
attributes. The COMMUNITIES attribute can be 32 bits long, with the
upper 16 bits indicating the AS number that was defined in the community.
The lower 16 bits have only local significance and are the community num-
ber. Enter the value as a single decimal number in the format AS:nn, where
is the AS number and nn is the lower 16-bit local community number. The
total community value is displayed as one long decimal number by default.
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