| This section describes the process
that BGP uses to make routing decisions. Routes are exchanged
between BGP peers via update messages. BGP routers receive the
update messages, run some policies or filters over the updates, and
then pass the routes on to other BGP peers.
The Cisco implementation of BGP keeps
track of all BGP updates in a BGP table separate from the IP routing
table. In case multiple routes to the same destination exist, BGP
does not flood its peers with all those routes. Instead, it picks
only the best route and sends it to the peers. In addition to passing along routes
from peers, a BGP router may originate routing updates to advertise
networks that belong to its own AS. Valid local routes originated in
the system and the best routes learned from BGP peers are then
installed in the IP routing table. The IP routing table is used for
the final routing decision.
The following sections detail the BGP
routing process, implementing BGP routing policy, controlling BGP
routing with attributes, and handling the BGP decision process.
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