The MPLS Fast Re-Route (FRR) functionality enables PLRs to be aware of the missing node protection and allows them to regularly probe for a node-bypass.
The following figure shows an LSP scenario.
Where:
LSP 1: between PE_1 to PE_2, with CSPF, FRR facility node-protect enabled.
P_1 protects P_2 with bypass-nodes P_1 -P_3 - P_4 - PE_4 -PE_2.
If P_4 fails, P_1 tries to establish the bypass-node three times.
When the bypass-node creation fails, P_1 will protect link P_1-P_2.
P_1 protects the link to P_2 through P_1 - P_5 - P_2.
P_4 returns online.
As LSP 1 had requested node protection, but because of a lack of any available path, it could only obtain link protection. Therefore, every 60 seconds the PLR for LSP 1 will search for a new path that may be able to provide node protection. When P_4 is back online and such a path is available, A new bypass tunnel will be signaled and LSP 1 will get associated with this new bypass tunnel.