The mpls traffic-eng fast-reroute backup-prot-preemption command allows you to determine the criteria the router will use when selecting the LSPs that will be preempted.
If you enter the command with the optimize-bw keyword, the router chooses LSPs that will waste the least amount of bandwidth.
If you do not enter the mpls traffic-eng fast-reroute backup-prot-preemption optimize-bw command, the router preempts as few LSPs as possible.
Each router in the network does not have to use the same algorithm; that is, you can specify optimize-bw for some routers in the network but not for others.
You can enter the mpls traffic-eng fast-reroute backup-prot-preemption command at any time. If you change the algorithm, it does not affect LSPs that already are protected. It only affects the placement of new LSPs signaled after you enter this command. The command can affect LSPs during the next periodic promotion cycle.
In the following examples, a next-next hop (NNHOP) backup tunnel has the following characteristics:
Total backup capacity: 240 units
Used backup bandwidth: 220 units
Available backup bandwidth: 20 units
The backup tunnel currently is protecting LSP1 through LSP5, which have the following bandwidth, and do not have backup bandwidth protection (that is, the “bandwidth protection desired” bit was not set via the tunnel mpls traffic-eng fast-reroute command):
LSP1: 10 units
LSP2: 20 units
LSP3: 30 units
LSP4: 60 units
LSP5: 100 units
As shown, LSP1 through LSP5 use 220 units of bandwidth.
LSP6 has backup bandwidth protection and needs 95 units of bandwidth. Twenty units of bandwidth are available, so 75 more units of bandwidth are needed.
In the following example, backup bandwidth protection is enabled and the amount of wasted bandwidth is minimized:
Router(config)# mpls traffic-eng fast-reroute backup-prot-preemption optimize-bw
LSP2 and LS4 are preempted so that the least amount of bandwidth is wasted.
In the following example, backup protection preemption is enabled and the number of preempted LSPs is minimized:
Router(config)# no mpls traffic-eng fast-reroute backup-prot-preemption
The router selects the LSP whose bandwidth is next-greater than the required bandwidth. Therefore, the router picks LSP5 because it has the next larger amount of bandwidth over 75. One LSP is demoted. and 25 units of bandwidth are wasted.