BGP Fast-External-Fallover Command Behavior

This feature allows the router to bring a BGP session down when the interface to that peer goes down. If you don’t want to allow this to be happen, then you can disable it.

This feature is supported only for directly connected eBGP peers. I have tested it with eBGP multihop peers but no result.

When, we are having disabled this feature. BGP peer will remain up till the BGP hold timer expires. That means keepalive is using to track the BGP session which is  default behavior.

You can use this feature per interface basis by configuring ip bgp fast-external-fallover  command under interface or you can use it for all the directly connected eBGP  peers by configuring BGP fast-external-fallover command under BGP.

By default this feature is enabled on all cisco routers. If required then we can disable it according our requirement. You can use below command to disable this feature.

R1#conf t
R1(config)#router bgp 100
R1(config-router)#no bgp fast-external-fallover
R1(config-router)#exit
R1(config)#

When should we disable  it?

We can disable BGP fast-external-fallover when connected interface is continuously flapping.

Below is our topology:

R1 is connected to R2 via interface fastethernet0/0. We will check the default behavior.

R1#sh ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.10.12.2 4 200 188 188 38 0 0 01:56:59 4

As you can see in above output from R1, BGP peering is up with R2. Now will shut the fastethernet0/0 on R1 which is connected to R2 and then we will check the default BGP behavior on R1.

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

*Dec 7 03:16:21.270: %BGP-5-ADJCHANGE: neighbor 10.10.12.2 Down Interface flap

Above output is showing that BGP peer  session was immediately went down just after shut down the connected interface.

Now we will re-enable the interface and then we will disable bgp- fast-external-failover and again shut the interface.

R1(config)#int f0/0
R1(config-if)#no shut

Now interface came up and BGP session has been re-established.

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.10.12.2 4 200 196 194 50 0 0 00:00:03 4

Now we will remove / disable fast-external-fallover and will shut the interface.

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)#

*Jan 4 03:19:41.386: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Jan 4 03:19:42.386: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
R1(config-if)#

Above logs are showing that interface has been administratively down but BGP session is still up.

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.10.12.2 4 200 196 194 54 0 0 00:00:50 4

BGP session is still up but interface is down. This BGP session will be down once the hold timer is expired.

R1#show ip bgp sum | be Ne
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.10.12.2 4 200 196 195 54 0 0 00:01:29 4