BGP fast external fallover uses the BGP protocol to converge quickly in the event of a link failure that connects it to an EBGP neighbor.
When BGP fast external fallover is enabled, the directly-connected EBGP neighbor session is immediately reset when the connecting link goes down and routes are learned over the BGP session. Failover is not initiated if routes are not learned over the BGP session.
If BGP fast external fallover is disabled, BGP waits until the default hold timer expires (3 keepalives) to reset the neighboring session. In addition, BGP may tear down the session somewhat earlier than hold timer expiry if BGP detects that the TCP session and it's directly connected link is broken (BGP detects this while sending or receiving data from the TCP socket).