BGP Prefix Advertisement

By entering network statements into the BGP configuration, it only identifies specific network prefixes to be advertised into the BGP table (Loc-RIB)

The network statement is under the address family configuration section of the BGP process configuration. A route-map can be used to tailor routes advertised or setting path attributes to neighbours.

The BGP process searches in the local RIB to find a match to any network statement configured. It can be for a connected, or not directly connected network from any routing protocol or statically configured.

If a match is found, it is advertised into the Loc-RIB, and some path attributes are set depending on the prefix type:

• If the prefix is a connected network, the next-hop BGP attribute is set to 0.0.0.0, origin attribute is set to IGP (i) and weight set to 32,768.
• If the prefix is a static or routing protocol network, the next-hop attribute is set to the next-hop IP address in the routing information base, origin set to IGP (i), weight set to 32,768 and the Multi-exit discriminator (MED) is set to the same metric as the IGP.

Every route in the LOC-RIB goes under a validity check, to ensure it is suitable for advertisement to other BGP peers:

• Verify that the network layer reachability information is correct and the next-hop address is resolvable in the global RIB.
• Process outbound neighbour route policies. If the route was not denied by outbound route policies, it can be maintined in the Adj-RIB-Out table for later use.
• Advertise the network reachability information (NLRI) to BGP peers. If the next-hop address is set to 0.0.0.0, then the next-hop address is changed to the IP address of the source of the BGP session.