There are a couple of things that affect the operation you're asking about:
- Detection of the changes: How fast can the router detect that the OSPF routes hanged? In general, this part depends on the BGP scanne, which checks he routing table once per minute.
- Propagation of the changes: How fast/often can BGP advertise the changes? The MinAdvertisemtInterval timr (by default 30 sec for eBGP and 5sec for iBGP) determines how often a train of updates can be sent to a peer.
In summary, the worst case (using default timers) would be anywhere from 5sec (iBGP) to 60 sec -- depending on when the change occurs. The bet case would be immediatelly. :-
--
Alvaro.