BGP fast-external-fallover

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.

Historically, when the fast-external-fallover feature was not available and a link went down, the EBGP session remained up until the hold-down timer expired. This situation used to cause a traffic black hole situation and service impact. To overcome this problem, bgp fast-external-fallover command was introduced. With this command configured, the EBGP session terminates immediately if the link goes down.

This feature is enabled by default for EBGP sessions but disabled for IBGP sessions.

Although the command bgp fast-external-fallover improves on convergence time, it is good to disable the command if the EBGP link is flapping continuously. By disabling fast-fallover, the instability caused by neighbors continually transitioning between idle and established states and the routing churn caused by the flood of ADVERTISE and WITHDRAW messages can be avoided.

Configuration

Purpose: Fast external fallover is enabled by default on IOS, IOS XR and NX-OS. When an interface that is used for a BGP connection goes down, the BGP session is immediately terminated. If the interface is flapping, instability can be caused, because the neighbors will constantly be transitioning between the idle and established states. There will also be a flood of BGP UPDATE and WITHDRAWN messages. If you have a flapping interface, use the no form of this command.

  • Use the no bgp fast-external-fallover command to disable this feature on both Cisco IOS and NX-OS.
  • Use the command bgp fast-external-fallover disable command to disable this feature on IOS XR.
  • The feature can also be enabled at the interface level using the command ip bgp fast-external-fallover on Cisco IOS.
				
					// no bgp fast-external-fallover (EBGP)
*Jul 22 04:16:46.232: %BGP-3-NOTIFICATION: sent to neighbor 10.3.3.1 4/0 (hold time expired) 0 bytes 
*Jul 22 04:16:46.233: %BGP-5-NBR_RESET: Neighbor 10.3.3.1 reset (BGP Notification sent)
*Jul 22 04:16:46.236: %BGP-5-ADJCHANGE: neighbor 10.3.3.1 Down BGP Notification sent
*Jul 22 04:16:46.236: %BGP_SESSION-5-ADJCHANGE: neighbor 10.3.3.1 IPv4 Unicast topology base removed from session  BGP Notification sent

// bgp fast-external-fallover enabled by default (EBGP)
*Jul 22 04:19:34.594: %BGP-5-NBR_RESET: Neighbor 10.3.3.1 reset (Interface flap)
*Jul 22 04:19:34.598: %BGP-5-ADJCHANGE: neighbor 10.3.3.1 Down Interface flap
*Jul 22 04:19:34.599: %BGP_SESSION-5-ADJCHANGE: neighbor 10.3.3.1 IPv4 Unicast topology base removed from session  Interface flap
*Jul 22 04:19:36.561: %LINK-5-CHANGED: Interface GigabitEthernet0/3, changed state to administratively down
*Jul 22 04:19:37.563: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/3, changed state to down

// bgp fast-external-fallover disabled by default (IBGP)
*Jul 22 04:25:09.181: %BGP-3-NOTIFICATION: sent to neighbor 10.1.1.1 4/0 (hold time expired) 0 bytes 
*Jul 22 04:25:09.182: %BGP-5-NBR_RESET: Neighbor 10.1.1.1 reset (BGP Notification sent)
*Jul 22 04:25:09.184: %BGP-5-ADJCHANGE: neighbor 10.1.1.1 Down BGP Notification sent
*Jul 22 04:25:09.185: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.1 IPv4 Unicast topology base removed from session  BGP Notification sent

// neighbor x.x.x.x fall-over configured (IBGP)
*Jul 22 04:29:51.308: %BGP-5-NBR_RESET: Neighbor 10.1.1.1 reset (Route to peer lost)
*Jul 22 04:29:51.311: %BGP-5-ADJCHANGE: neighbor 10.1.1.1 Down Route to peer lost
*Jul 22 04:29:51.312: %BGP_SESSION-5-ADJCHANGE: neighbor 10.1.1.1 IPv4 Unicast topology base removed from session  Route to peer lost
*Jul 22 04:29:53.273: %LINK-5-CHANGED: Interface GigabitEthernet0/1, changed state to administratively down
*Jul 22 04:29:54.273: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down
				
			
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