The MPLS Fast Re-Route (FRR) feature implements a background task to evaluate Path State Block (PSB) associations with bypass LSP. The following is the task evaluation behavior:
For PSBs that have facility FRR enabled but no bypass association, the task triggers a FRR protection request.
For PSBs that have requested node-protect bypass LSP but are currently associated with a link-protect bypass LSP, the task triggers a node-protect FRR request.
For PSBs that have LSP statistics enabled but the statistic index allocation failed, the task re-attempts index allocation.
The MPLS FRR background task enables PLRs to be aware of the missing node protection and allows them to probe regularly for a node-bypass. Figure: FRR node-protection example shows an example of FRR node protection.
The following describes an LSP scenario where:
LSP 1: from 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 protects link P_1-P_2
P_1 protects the link to P_2 through P_1 - P_5 - P_2
P_4 returns online
LSP 1 has requested node protection, but because there is no available path, it can only obtain link protection. Therefore, every 60 seconds the PSB background task triggers the PLR for LSP 1 to search for a new path that can provide node protection. When P_4 is back online and such a path is available, a new bypass tunnel is signaled and LSP 1 gets associated with this new bypass tunnel.