How to Calculate Fast Re Route(FRR) - Repair Path

The goal of IP Fast Reroute (IP FRR) is to reduce failure reaction time to sub-50ms by using a pre-computed alternate next-hop, in the event that the currently selected primary next-hop fails, so that the alternate can be rapidly used when the failure is detected. The alternate path or repair path must be loop free path to destination in case of network failures without recomputing the network topology as per below mentioned design.



Repair path can be used for diverting live traffic flowing through the primary path protected by the repair path only if the repair path both bypasses local failure (for example, failing link) and does not cause traffic to loop. Such repair paths are called “loop-free alternatives” or LFA.

Repair path provides loop-free alternative if following criterion is met:
1. Loop Free Alternate:- Condition: D(N,D) < D(N,S) + D(S,D). Path is loop-free because N’s best path is not through local router.” Traffic sent to backup next hop is not sent back to S.
2. Downstream Path:- Condition D(N,D) < D(S,D). “Neighbor router is closer to the destination than local router.” Loop-free is guaranteed even with multiple failures (if all repair-paths are downstream path).
3. Node protection:- Condition D(N,D) < D(N,E) + D(E,D). “N's path to D must not go through E.” “The distance from the node N to the prefix via the primary next-hop is strictly greater than the optimum distance from the node N to the prefix.“

It will be more cleared with below stated example:-


Examples of Calculating Fast Re-Route

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