Default IP routes are advertised by border routers to its peers. So this allows peers to use the advertised router as a next hop for destinations which are not listed in the routing table. This feature allows routers which are not able to maintain a huge list of routing entries in their routing table to use the default traffic to the advertised border router. You do not have to configure the static routes on the routers which are not able to maintain the routing table entries.
The IP Manager monitors the default route entry on the border router, that is, the border routers are polled to check only the default route entry and not the entire routing table, and when the default route entry is missing, the IP Manager generates an event known as MissingDefaultRoute. This reduces the amount of SNMP request processing that the router has to perform.
The OID which is used for this feature is ipRouteDest-18.104.22.168.22.214.171.124.1.1. Normally the default IP route is 0.0.0.0 which gets advertised using the update message by either the border router speaker to all its peers. The update message also contains the next hop attribute which is the IP address of the border router. That is how the peers know about the border router speaker.