Cisco BGP-4 Command and Configuration Handbook

3-15 bgp fast-external-fallover

Syntax Description:

This command has no arguments.

Purpose: Fast external fallover is enabled by default. 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 There will also be a flood of BGP UPDATE and WITHDRAWN messages.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.

Cisco IOS Software Release: 11.0

Configuration Example: Demonstration of Fast External Fallover

This configuration example demonstrates using the bgp fast-external-fallover command. In Figure 3-11, the Ethernet interface on Router B is flapping. We will investigate the result of using both fast-external-fallover and no fast-external-fallover.

Router A router bgp 1 neighbor 172.17.1.2 remote-as 2 ___________________________________________________________________________ Router B router bgp 2 fast-external-fallover neighbor 172.17.1.1 remote-as 1

Verify that Routers A and B have formed a BGP connection:

rtrB# show ip bgp neighbors BGP neighbor is 172.17.1.1, remote AS 1, external link Index 1, Offset 0, Mask 0x2 BGP version 4, remote router ID 172.17.1.1 BGP state = Established, table version = 58, up for 00:14:02

Enable event debugging on Router B using the command debug ip bgp events. In configuration mode on Router B, shut down the Ethernet interface and observe the debug output:

1d17h: BGP: 172.17.1.1 reset requested 1d17h: BGP: 172.17.1.1 reset due to Interface flap 1d17h: BGP: 172.17.1.1 went from Established to Idle

Using the default bgp fast-external-fallover, you can see that when the interface goes down, the BGP session is immediately reset. Enable the Ethernet interface on Router B, and configure the no form of this command on Router B:

Router B router bgp 2 no bgp fast-external-fallover neighbor 172.17.1.1 remote-as 1

Verification

After Routers A and B have established a BGP connection, shut down the Ethernet interface on Router B, and observe the debug output:

1d17h: %LINK-5-CHANGED: Interface Ethernet0, changed state to administratively down 1d17h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to down

The BGP session did not drop, and the BGP neighbor relationship remained in the established state. Of course, if the interface remains down longer than the configured BGP hold time, the connection is dropped:

1d17h: BGP: 172.17.1.1 reset due to Peer timeout 1d17h: BGP: 172.17.1.1 went from Established to Idle

But if the interface goes up before the hold time expires , the session is not terminated.

Troubleshooting

Step 1. Verify that the BGP neighbors are in the Established state using the show ip bgp neighbors command.

If the neighbor relationship is not in the Established state, see section 8-23. For IBGP and loopbacks, see section 8-33.

Step 2. Verify that the desired form of the bgp fast-external-fallover command has been configured. By default, bgp fast-external-fallover is enabled.