BGP is used to transmit routing information. BGP advertises only the wanted routes which match a routing policy and modifies route attributes to direct network traffic properly.
BGP is used to transmit routing information between ASs. Route advertisement directly affects traffic forwarding.
There are usually a large number of routes in a BGP routing table. Transmitting a great deal of routing information intensifies the load on devices. To address this issue, control routes to be advertised. You can configure devices to advertise only routes that these devices want to advertise or routes that their peers require.
In addition, multiple routes that are destined for the same IP address but traverse different ASs may exist.
To direct traffic to specific ASs, filter these routes before advertising them. BGP can filter routes to be advertised to a specific peer or peer group. If multiple filter policies are configured, BGP advertises only routes that match all the filter policies.