The MPLS Fast Reroute (FRR) functionality enables PLRs to be aware of the lack of node protection and lets them regularly probe for a node bypass via the node-protect command.
When enabled, the node-protect command provides node protection for the specified LSP. If node protection cannot be provided, link protection is attempted. If link protection cannot be provided, no protection is provided. When disabled via the no form of the command, link protection is attempted, and if link protection cannot be provided, no protection is provided.
For example, assume the following for the LSP scenario in Figure: FRR Node-Protection Example.
LSP_1 is between PE_1 and PE_2 (via P1 and P2), and has CSPF, FRR facility backup, and FRR node protection enabled.
P1 protects P2 with bypass nodes P1 - P3 - P4 - PE_4 - PE_3.
If P4 fails, P1 tries to establish the bypass node three times.
When the bypass node creation fails (there is no bypass route), P1 will protect link P1-P2.
P1 protects the link to P2 through P1 - P5 - P2.
P4 returns online.
LSP_1 had requested node protection, but due to lack of an available path it could only obtain link protection. Therefore, every 60 s, the PLR for LSP_1 will search for a new path that might be able to provide node protection. When P4 is back online and such a path is available, a new bypass tunnel will be signaled and LSP_1 will be associated with this new bypass tunnel.