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Routing & Switching · Advanced

Loop-Free Alternate (LFA)

A precomputed backup next hop that is guaranteed loop-free, enabling sub-50 ms IGP fast reroute.

Definition

A neighbor qualifies as an LFA if its shortest path to the destination does not pass back through the protecting router (dist(N,D) < dist(N,S) + dist(S,D)). LFAs are installed in the FIB as backups, but coverage depends on topology. Remote LFA and TI-LFA extend coverage using tunnels or segment routing.

Example

fast-reroute per-prefix enable prefix-priority low under router ospf 1

Exam tip

LFA coverage is topology dependent; TI-LFA provides 100 percent coverage with segment routing.

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