James,
BGP keepalives are important to validate BGP sessions health and to avoid routing blackholes.
if you are setting your keepalive interval & holddown timers in production you need to be very careful. Setting the timers too small results in fast peering deactivateion detection and lead to false positives and disruption of BGP session and exessibe route flapping,
For fast-external-failover, this feature is enable by default for eBGP sessions using the BGP process command bgp fast-external-failover and used for point-point links on a none-shared link when the peers are directly connected. Using it on NBMA and ethernet interfaces might be inefficient.
A more advance version is called fast peering session deactivation which can be setup using the neighbor command "neighbor 1.1.1.1 fall-over. This feature applies to both iBGP & eBGP and does not rely on interface state instead uses IGP. Once the IGP route disappear, the BGP session gets immediately torn down and used for none-direct conencted peers.
Francisco
