Another great article from Ivan Pepelnjak.
What was particularly interesting to me is the idea to check presence of specific routes in the routing table before advertising default route!
This kind of tracking can also be used to quickly close peering session with a neighboring BGP router. Imagine we have a host route to a BGP peer loopback that is advertised, for example, via OSPF. We can match this route with a prefix-list and then use it in a route-map to create a condition for a BGP session closing. This described in a great detail in Designing Fast Converging BGP Networks.
