MPLS Resilience, Fast Reroute and Protection
What this domain tests
MPLS Resilience, Fast Reroute and Protection tests how MPLS recovers from link and node failures before IGP reconvergence. It covers RSVP-TE FRR, facility backup, one-to-one protection, LFA, and BFD.
Core rules
- FRR provides local repair at a point of local repair (PLR). The PLR pre-signals a backup tunnel and redirects traffic when the protected link or node fails.
- Facility backup protects a whole link or node by using a bypass tunnel; one-to-one backup creates a dedicated backup LSP for each protected LSP.
- The merge point (MP) receives rerouted traffic and continues forwarding. The backup path must avoid the protected resource.
- LFA uses the IGP topology to precompute a loop-free alternate. Remote LFA can tunnel repair traffic to a PQ node when no local LFA exists.
- BFD provides fast failure detection; MPLS OAM can verify LSP continuity and path.
Worked example
PE1-P1-P2-PE2 carries a primary TE tunnel. P1 is the PLR for the P1-P2 link. P1 has a bypass tunnel to P2's next hop, P3. When the P1-P2 link fails, P1 detects it, pushes the bypass label, and forwards traffic to P3. P3 swaps to the bypass and delivers traffic to P2. The primary headend may not know until RSVP/IGP reconverges. The reasoning is local repair: P1 acts before the network-wide path changes.
Traps
Do not confuse FRR with IGP convergence. Do not assume a bypass tunnel protects every failure. Do not forget that the backup path must not use the protected link, and that PHP can change label operations at the merge point.