BGP Fast-external-fallover command terminates external BGP sessions of any directly adjacent peer if the link used to reach the peer goes down; without waiting for the hold-down timer to expire.
Although this feature improves the BGP conversion time, it may lead to great instability in your BGP table due to a flapping interface. By default this feature is enabled in Cisco IOS.
In the following topology R1 & R2 configured with an EBGP session between them as follows:
On Both routers you can see BGP neighborship is up
When you shut down the interface connected between R1 & R2; you can see R1 immediately resets the peering session without waiting for the hold-down time to expire as shown in the debug output below:
From the above output you can see neighbors go down immediately
I have no shut the interface and again we have bgp neighborship. Now let’s disable “bgp fast-external-fallover” command, shutdown the interface and see what will happen?
From the above output you can see that after shutting the interface connected between R1 & R2 down the session remains up until the hold-down timer expires.
BGP fast external fallover can also be configured on a per-interface basis using the "ip bgp fast-external-fallover" interface configuration command.