The default route origination and advertisement feature allows you to originate and advertise a default route to a BGP neighbor (or to all neighbors in a peer group) even though no default route exists in the local IP routing table. It also allows you to associate policy rules to conditionally advertise a default route to BGP neighbors.
When default route origination becomes active, the default route is advertised to the specified BGP neighbors, overriding any previously sent default route. If a default route is added to the local IP routing table while default route origination is active, the default route defined by this feature takes precedence over the new regular default route. If default route origination becomes inactive, and a regular default route exists, the regular default route is advertised to BGP neighbors.
After a policy is configured for default route origination, BGP must periodically scan the local BGP RIB to make sure that the policy rules evaluate to true for at least one route in local BGP RIB. If the rules evaluate to true, default origination remains active. If the rules evaluate to false, then default origination becomes inactive and the default routes must be withdrawn.
For more information on policy match conditions, actions, and configuration, see Routing Policies.