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Configuring the Smart Optimize Timer for LSPs

 

Because of network and router resource constraints, it is typically inadvisable to configure a short interval for the optimize timer. However, under certain circumstances, it might be desirable to reoptimize a path sooner than would normally be provided by the optimize timer.

For example, an LSP is traversing a preferred path that subsequently fails. The LSP is then switched to a less desirable path to reach the same destination. Even if the original path is quickly restored, it could take an excessively long time for the LSP to use it again, because it has to wait for the optimize timer to reoptimize the network paths. For such situations, you might want to configure the smart optimize timer.

When you enable the smart optimize timer, an LSP is switched back to its original path so long as the original path has been restored within 3 minutes of going down. Also, if the original path goes down again within 60 minutes, the smart optimize timer is disabled, and path optimization behaves as it normally does when the optimize timer alone is enabled. This prevents the router from using a flapping link.

The smart optimize timer is dependant on other MPLS features to function properly. For the scenario described here in which an LSP is switched to an alternate path in the event of a failure on the original path, it is assumed that you have configured one or more of the MPLS traffic protection features, including fast reroute, link protection, and standby secondary paths. These features help to ensure that traffic can reach its destination in the event of a failure.

At the least, you must configure a standby secondary path for the smart optimize timer feature to work properly. Fast reroute and link protection are more temporary solutions to a network outage. A secondary path ensures that there is a stable alternate path in the event the primary path fails. If you have not configured any sort of traffic protection for an LSP, the smart optimize timer by itself does not ensure that traffic can reach its destination. For more information about MPLS traffic protection, see MPLS and Traffic Protection.

When a primary path fails and the smart optimize timer switches traffic to the secondary path, the router might continue to use the secondary path even after the primary path has been restored. If the ingress router completes a CSPF calculation, it might determine that the secondary path is the better path.

This might be undesirable if the primary path should be the active path and the secondary path should be used as a backup only. Also, if the secondary path is being used as the active path (even though the primary path has been reestablished) and the secondary path fails, the smart optimize timer feature will not automatically switch traffic back to the primary path. However, you can enable protection for the secondary path by configuring node and link protection or an additional standby secondary path, in which case, the smart optimize timer can be effective.

Specify the time in seconds for the smart optimize timer using the smart-optimize-timer statement:

You can include this statement at the following hierarchy levels:

  • [edit protocols mpls]

  • [edit logical-systems logical-system-name protocols mpls]