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BGP receives routes from external or
internal peers. Depending on what is configured in the input policy
engines, some or all of these routes will make it into the router
BGP table.
Input Policy Engine
This engine handles route filtering
and attribute manipulation. Filtering is done based on parameters
such as the IP prefix, the AS path, and attribute information.
The network administrator can use the
input policy engine to manipulate the path attributes and influence
BGP's decision process, which affects what routes it will actually use
to reach a certain destination. If, for example, the network
administrator chooses to filter a certain network coming from a
peer, it is an indication to BGP that it should not reach that
network via that peer. Or, if a network administrator gives a
certain route a better local preference than some other path to the
same destination (this attribute is discussed later), it is an
indication that BGP should prefer this route over the other
available routes.
The Decision Process
BGP goes through a decision process
to decide which routes it wants to use to reach a certain
destination. The decision process is based on the routes that made
it into the router after the input policy engine was applied and is
performed on the routes in the BGP routing table. The decision
process looks at all the available routes for the same destination,
compares the different attributes associated with each route, and
chooses one best route. The decision process is discussed later in
this chapter.
Routes Used by the Router
The best routes, as
identified by the decision process, are candidates to be advertised
to other peers. These routes are also presented to the routing
engine to be placed in the IP routing table (not all routes
presented to the routing engine will be placed in the routing table,
since multiple protocols may present he same prefix for
installation, and the router must choose between them).
In addition to routes passed on from
other peers, the router (if configured to do so) originates updates
about the networks inside its AS. This is how an AS injects its
routes into the outside world.
Output Policy Engine
This is the same engine as the input
policy engine, applied on the output side. Routes used by the router
(the best routes) in addition to routes that the router generates
locally are given to this engine for processing. The engine might
apply filters and might change some of the attributes (such as
AS_path or metric) before sending the update.
The output policy engine also
differentiates between internal and external peers; for example,
routes learned from internal peers cannot be passed on to internal
peers.
Routes Advertised to Peers
The routes advertised to peers are
routes that made it through the output engine, and they are
advertised to the BGP peers, internal or external.
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