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Configuring MPLS Soft Preemption


Soft preemption attempts to establish a new path for a preempted LSP before tearing down the original LSP. The default behavior is to tear down a preempted LSP first, signal a new path, and then reestablish the LSP over the new path. In the interval between when the path is taken down and the new LSP is established, any traffic attempting to use the LSP is lost. Soft preemption prevents this type of traffic loss. The trade-off is that during the time when an LSP is being soft preempted, two LSPs with their corresponding bandwidth requirements are used until the original path is torn down.

MPLS soft preemption is useful for network maintenance. For example, you can move all LSPs away from a particular interface, then take the interface down for maintenance without interrupting traffic. MPLS soft preemption is described in detail in RFC 5712, MPLS Traffic Engineering Soft Preemption.

Soft preemption is a property of the LSP and is disabled by default. You configure it at the ingress of an LSP by including the soft-preemption statement:

You can include this statement at the following hierarchy levels:

You can also configure a timer for soft preemption. The timer designates the length of time the router should wait before initiating a hard preemption of the LSP. At the end of the time specified, the LSP is torn down and resignaled. The soft-preemption cleanup timer has a default value of 30 seconds; the range of permissible values is 0 through 180 seconds. A value of 0 means that soft preemption is disabled. The soft-preemption cleanup timer is global for all LSPs.

Configure the timer by including the cleanup-timer statement:

You can include this statement at the following hierarchy levels:


Soft preemption cannot be configured on LSPs for which fast reroute has been configured. The configuration fails to commit. However, you can enable soft preemption in conjunction with node and link protection.


The counter value for SoftPreemptionCnt initializes with a value of 0 (zero), visible in the command show rsvp interface detail output.