Configure Priority and Preemption

When there is insufficient bandwidth to establish a more important LSP, you might want to tear down a less important existing LSP to free up the bandwidth. You do this by preempting the existing LSP.

Whether an LSP can be preempted is determined by two properties associated with the LSP:

• Setup priority—Determines whether a new LSP that preempts an existing LSP can be established. For preemption to occur, the setup priority of the new LSP must be higher than that of the existing LSP. Also, the act of preempting the existing LSP must produce sufficient bandwidth to support the new LSP. That is, preemption occurs only if the new LSP can be set up successfully.
• Hold priority—Determines the degree to which an LSP holds onto its session reservation after the LSP has been set up successfully. When the hold priority is high, the existing LSP is less likely to give up its reservation and hence it is unlikely that the LSP can
  be preempted.

You cannot configure an LSP with a high setup priority and a low hold priority because permanent preemption loops might result if two LSPs are allowed to preempt each other. You must configure the hold priority to be higher than or equal to the setup priority.

The setup priority also defines the relative importance of LSPs on the same ingress router. When the software starts, when a new LSP is established, or during fault recovery, the setup priority determines the order in which LSPs are serviced. Higher priority LSPs tend to be established first and hence enjoy more optimal path selection.

To configure the LSP's preemption properties, include the priority statement at the [edit protocols mpls], [edit protocols mpls label-switched-path lsp-path-name ], or [edit protocols mpls label-switched-path lsp-path-name (primary | secondary)] hierarchy level:

priority setup-priority hold-priority ; 

Both setup-priority and hold-priority can be a value from 0 through 7. The value 0 corresponds to the highest priority, and the value 7 to the lowest. By default, an LSP has a setup priority of 7 (that is, it cannot preempt any other LSPs) and a hold priority of 0 (that is, other LSPs cannot preempt it). These defaults are such that preemption does not happen. When you are configuring these values, the setup priority should always be less than or equal to the
hold priority.