LSP 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, and 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 can enable it at the ingress of an LSP by using the soft-preemption statement.