BGP route advertisement rules
BGP follows these rules for route advertisement:
When multiple feasible routes to a destination exist, BGP advertises only the optimal route to its peers. If the advertise-rib-active command is configured, BGP advertises the optimal route in the IP routing table. If not, BGP advertises the optimal route in the BGP routing table.
BGP advertises only routes that it uses.
BGP advertises routes learned from an EBGP peer to all BGP peers, including both EBGP and IBGP peers.
BGP advertises routes learned from an IBGP peer to EBGP peers, rather than other IBGP peers.
After establishing a session to a new BGP peer, BGP advertises all the routes matching the above rules to the peer. After that, BGP advertises only incremental updates to the peer.