Fast External Neighbor Loss Detection – where when a directly conected BGP neighbors link goes down, IOS will immediately bring down that neighbor and flush all learned BGP routes from that neighbor. Causing it to find better routes elsewhere. This works for directly connected eBGP neighbors only.
Internal Neighbor Loss Detection – iBGP neighbors had no fast way to reconverge until IOS 12.0 when the neighbor fall-over command was introduced which allows iBGP neighbors to converge fast upon detecting the loss of a route to that neighbor. With that command configured upon losing a route, the router will immediately tear down the BGP session with the peer resulting in fast convergence. Proper failover with this setup though relies on the IGPs ability to supply a backup route.
EBGP Fast Session Deactivation – is the same as neighbor fall-over except its configured globally and applies to eBGP neighbors.