Route advertisement (based on BGP table)

BGP Route advertisement rules


Looking at the BGP table only Best ruotes are examined for advertisement 
(because they will be in the routing table) the rest of the routes can be ignored.


Cisco IOS only consider routes in the routing table for advertisement.

Router to IBGP peer (Route-NH-LP-MED-AS Path)

1. Only Best routes are candidate for advertisement.
2. Connected routes always advertised.
2. If next-hop-self is not set the route will have next hop of whatever is in the BGP table.
3. If the route has a next hop of an IBGP peer it is not advertised.
4. If the route has a next hop of an EBGP peer it is advertised to the IBGP peer even if it is also learned from that IBGP peer.

Router to EBGP peer (Route-NH-AS Path)

1. Only Best routes are candidate for advertisement.
2. Connected routes always advertised.
3. Next hop is always set to self.
4. If the best route has a next hop of that specific EBGP peer it is not advertised back (split horizon).