3.7.d Implement, optimize and troubleshoot routing policies

3.7.d [ii] Conditional advertisement

Normally, routes are propagated regardless of the existence of a different path. The BGP conditional advertisement feature uses the non -exist-map and the advertise-map keywords of the neighbor advertise-map command in order to track routes by the route prefix. If a route prefix is not present in output of the non-exist-map command, then the route specified by the advertise-map command is announced.

This feature is useful for multi-homed networks, in which some prefixes are advertised to one of the providers only if information from the other provider is not present (this indicates a failure in the peering session or partial reachability). The conditional BGP announcements are sent in addition to the normal announcements that a BGP router sends to its peers.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3975-3982).  . Kindle Edition.

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/16137-cond-adv.html