RSVP soft preemption implements a suite of protocol modifications extending the concept of preemption with the goal of reducing or eliminating traffic disruption of TE LSPs. It is achieved by using additional signaling and maintenance mechanisms to alert the ingress LER of the preemption that is pending and allows for temporary control-plane under-provisioning while the preempted tunnel is rerouted in a non-disruptive fashion (make before-break) by the ingress LER. During the period that the tunnel is being rerouted, link capacity is under-provisioned on the midpoint where preemption was initiated and potentially one or more links upstream along the path where other soft preemptions may have occurred. Soft preemption is a property of the LSP and is disabled by default.
The default preemption in an MPLS-TE network is hard preemption. This is helpful in cases where actual resource contention happens in the network. Soft Preemption provides flexibility for operators to select the type of preemption based on network conditions.
MPLS soft preemption is useful for network maintenance. For example, all LSPs can be moved away from a particular interface, and then the interface can be taken down for maintenance without interrupting traffic. MPLS soft preemption is also useful in dynamic networks where preemption often occurs.
Only adaptive and non-FRR LSPs could be enabled for soft preemption. LSPs which are adaptive and without FRR configuration have the facility to enable or disable the soft preemption feature without disabling the LSP. When the soft preemption configuration is changed, RSVP is notified for this change and a new Path message is triggered with the soft preemption desired flag bit (0x40) set in session attribute for signaling.