The Border Gateway Protocol (BGP) conditional advertisement feature provides greater control over route advertisements by permitting certain routes to be advertised based on the existence of other routes in the BGP table. This is a useful feature for multi-homed networks when it is desirable to advertise certain networks to one of the Internet Service Providers (ISPs), but only if the connectivity to another ISP fails.
To configure the BGP conditional advertisement feature, perform these steps:
Create a route map that permits a prefix the BGP speaker can track. Conditional advertisement takes place if the route being tracked does not exist in the BGP table. Create another route map that permits the prefix to be advertised. The routes that are conditionally advertised should also exist as valid routes in the BGP table. To create a route map, issue the route-map command in global configuration mode. Issue the match and set commands in route-map configuration mode.
Configure a conditional advertisement by binding the configured route maps to a BGP neighbor. This is done by issuing the neighbor advertise-map non-exist-map command in router configuration mode. The route map which specifies the tracked prefix is associated with the non-exist-map keyword. The route map which specifies the advertised prefix conditionally is associated with the advertise-map keyword.