Preemption is a mechanism that allows a higher priority LSP to tear down an existing lower priority LSP in order to obtain the required resources for its establishment. Preemption can occur when there is not enough bandwidth or other resources available on a link or node to accommodate a new LSP request. In this case, the node can select one or more lower priority LSPs that are using the resources and send them a PathErr message with a Preempt error code. This causes the lower priority LSPs to be terminated and release their resources.The node can then allocate the resources to the higher priority LSP and send a Resv message to confirm its reservation34.Reference:

3: RFC 4829: Label Switched Path (LSP) Preemption Policies for MPLS Traffic Engineering4

4: MPLS Applications User Guide | Juniper Networks5