Convergence, Fast Reroute and LFA / TI-LFA
Convergence, Fast Reroute and LFA / TI-LFA
This domain tests pre-computed repair paths: how a loop-free alternate is qualified, when no LFA exists, and how remote LFA and TI-LFA fill the gap.
Core rules
- An LFA is a neighbour that reaches the destination without looping back through the protected router. The loop-free condition is that the neighbour's distance to the destination is less than its distance to the protecting router plus the protected router's distance to the destination.
- LFA protects against a single next-hop or link failure and requires at least one qualifying neighbour; on ring and hub topologies it often fails.
- Remote LFA extends the search to a PQ node reached through a tunnel, typically an LDP or SR tunnel, so topologies without a direct LFA still get repair.
- TI-LFA uses segment routing to build an explicit repair path to the post-convergence next hop, giving near-100% coverage without extra protocols.
- On SR OS, LFA is enabled with
loopfree-alternateunder the OSPF instance, withremote-lfaand segment-routing options for the extended variants. Verify installed repairs in the OSPF route output.
Worked example
R1 protects prefix P behind R2. R1's other neighbour R3 reaches P at cost 30 and R2 at cost 20; R2 reaches P at cost 10. The condition 30 < 20 + 10 is false, so R3 is not an LFA. A remote LFA tunnel to a PQ node, or TI-LFA, provides the repair instead.
Traps that cost marks
- Assuming every topology has an LFA; a single-homed ring spur often does not.
- Computing the inequality the wrong way round.
- Expecting LFA to protect against more than one failure at a time.
- Enabling the feature but not checking that repairs are actually installed.