BGP - fast-external-fallover

This feature allows the router to bring a BGP session down when the interface to that peer goes down. If you don't want this or are asked to not allow this to happen, you can disable it:

R1 has a neighbor:

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
136.10.12.2 4 200 188 188 38 0 0 01:56:59 4

R1(config)#int f0/0
R1(config-if)#shut

*Dec 7 03:16:21.270: %BGP-5-ADJCHANGE: neighbor 136.10.12.2 Down Interface flap

We can prevent R1 from tearing the session down by disabling fast-external-fallover:

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
136.10.12.2 4 200 196 194 50 0 0 00:00:03 4

R1(config)#router bgp 100
R1(config-router)#no bgp fast-external-fallover
R1(config-router)#int f0/0
R1(config-if)#shut
R1(config-if)#

*Dec 7 03:19:41.386: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Dec 7 03:19:42.386: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
R1(config-if)#^Z

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
136.10.12.2 4 200 196 194 54 0 0 00:00:50 4

Still up:

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
136.10.12.2 4 200 196 195 54 0 0 00:01:29 4

Now the session will come down when the hold time expires. Some things to remember:

-Only works for directly-connected EBGP peers (hence the word "external" in the command)
-I tested with ebgp-multihop peers and it does not have any effect
-Keepalives are use to bring session down
-Also configurable per-interface with ip bgp fast-external-fallover