Understanding the network.pdf - Back to Home

Routers asbr-a1/a2 and asbr-b1/b2 are exchanging external BGP and internal BGP
routing information about their local networks. As you can see, both IBGP and EBGP
peer sessions (also referred to as speakers) are configured using the command. What demonstrates the existence of
IBGP peer session is that the remote AS number is identical to that of the local AS
number. Because there are four IBGP routers in AS-30001 that have EBGP peer
sessions and provide similar network reachability information, administrative
weights are used to set preferences for which routes the speakers'broadcasts will be
sent to.
NOTE
To prevent routing loops, external network information learned from IBGP peers is
not readvertised to other IBGP peers.
Weights are used to ensure that the external routes provided through the locally
adjacent peer are preferred over the remote peer. This same approach is used with
the IBGP routers using a special kind of access list for filtering as-path information.
as-path acl is created in general configuration EXEC mode using . The filter is then applied
to a neighbor using the .
Because all the AS-30001 routers are using similar configurations, a BGP
peer-group could be created to reduce some of the configuration overhead. BGP
peer-groups provide a way of configuring a core set of BGP parameters that are
used by all of the peer-group members. Peer-groups are created in the BGP
configuration subprocess:
asbr-b2(config-router)#neighbor as30001 peer-group
asbr-b2(config-router)#neighbor as30001 remote-as 30001