Configuring a Path Selection Revert Timer

Without a configured path selection revert timer, the router switches between a primary and secondary path immediately after the current working path goes down. A problem with this mode of operation is that it can cause flapping when the current path goes up and down frequently. Also, the LSP to which the route is switching traffic might be unstable, which causes the router to fail back to the current LSP almost immediately.

The path revert timer insures the stability of the LSP to which the traffic is switched by specifying the number of seconds that the LSP must be running before it carries traffic.

To configure a path selection revert timer for an LSP, complete the following steps using the revert-time command.

Enable the device for configuration.
```
device# configure
```
Enable the MPLS router.
```
device(config)# router mpls
```
Enable and specify signaled label switched path (LSP).
```
device(config-router-mpls)# lsp samplelsp
```
In this example, samplelsp is the selected LSP.
Configure revert timer.
```
device(config-router-mpls-lsp-samplelsp)# revert-timer 10
```
The timer-value value is the number of seconds that the router waits after the primary, or selected path comes up before traffic reverts to that path. in this example, the value is configured to 10 seconds.

Example

The following example configures the path selection revert time to 10 seconds for LSP named 'samplelsp'.

device# configure terminal
device(config)# router mpls
device(config-router-mpls)# lsp samplelsp
device(config-router-mpls-lsp-samplelsp)# revert-timer 10

When deploying the revert-time command, consider the following:

The revert-time command has no effect on the unconditional select mode. Traffic is unconditionally switched to the user-selected path and stays on it.
The path stability test used with the revert timer is based on the uptime of the latest instance of the path. This value can be different when the selected path has gone through a "make-before-break" procedure.
For an LSP going through re-optimization, the new LSP does not carry traffic until the revert timer expires.
When a user changes the revert time, the basis of counting is the uptime of the path and is independent of the sequence or combination of configurations. Take, for example, a path that is configured in the manual select mode to be a secondary path with a revert-time of 10 seconds. After the secondary path comes up, a 10-second timer starts, but after five seconds, the user changes the revert-timer value to four. Now the path has already been stable beyond the newly configured revert time, so the original time is canceled, and traffic immediately switches over. However, if the user were to change the revert-time value to eight seconds after running for five seconds, the existing count would terminate and start a new count of three seconds from the moment the first count terminated.