Overview of “bgp fast-external-fallover”

BGP neighbors are maintained by periodically exchanging KEEPALIVE messages. By default, KEEPALIVE messages are sent every 60 seconds, and if a KEEPALIVE message is not received for 180 seconds, the neighbor is considered down. “bgp fast-external-fallover” is a feature that immediately drops the neighbor if the interface used for the BGP connection goes down. The command syntax for “bgp fast-external-fallover” is as follows

bgp fast external-fallover

Router(config)#router bgp <AS>
Router(config-router)#bgp fast-external-failover

<AS> : ASnumber

bgp fast external-fallover behavior

Verify bgp fast-external-fallover behavior in the following network diagram.

Figure bgp fast-external-fallover
Figure bgp fast-external-fallover

R1 uses the IP address of Fa0/0 to establish a BGP neighbor with ISP1.

When “bgp fast-external-fallover” is enabled

bgp fast-external-failover is enabled by default. Shut down Fa0/0 with debug ip bgp events enabled on R1.

R1#debug ip bgp events
BGP events debugging is on
R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#int fa 0/0
R1(config-if)#shutdown
R1(config-if)#end
R1#
*Mar  1 00:12:24.712: BGP: service reset requests
*Mar  1 00:12:24.712: BGP: service reset requests
*Mar  1 00:12:24.712: BGP: service reset requests
*Mar  1 00:12:24.712: BGP: service reset requests
*Mar  1 00:12:24.712: BGP: 172.16.1.11 reset due to Interface flap
R1#
*Mar  1 00:12:24.712: %BGP-5-ADJCHANGE: neighbor 172.16.1.11 Down Interface flap
*Mar  1 00:12:25.629: %SYS-5-CONFIG_I: Configured from console by console
R1#
*Mar  1 00:12:26.707: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Mar  1 00:12:27.709: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down

If you shut down Fa0/0 on R1, you will immediately see that the neighbor with ISP1 is disconnected.

When “bgp fast-external-fallover” is disabled

Then disable fast-external-fallover on R1 and shut down Fa0/0.

R1(config)#router bgp 100
R1(config-router)#no bgp fast-external-fallover
R1(config-router)#exit
R1(config)#int fa 0/0
R1(config-if)#shutdown
R1(config-if)#end
R1#
*Mar  1 00:19:24.031: %SYS-5-CONFIG_I: Configured from console by console
R1#
*Mar  1 00:19:25.169: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Mar  1 00:19:26.171: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
R1#
*Mar  1 00:21:48.716: %BGP-5-ADJCHANGE: neighbor 172.16.1.11 Down BGP Notification sent
R1#
*Mar  1 00:21:48.716: %BGP-3-NOTIFICATION: sent to neighbor 172.16.1.11 4/0 (hold time expired) 0 bytes

Disabling bgp fast-external-fallover shows that shutting down Fa0/0 does not immediately bring down the neighbor with ISP1, but maintains the neighbor until the hold time has passed.

How the BGP works