Add up BFD detection, routing convergence and application recovery to get real WAN failover time and downtime.
Failover is a sum of serial delays: the liveness protocol must first declare the path dead, the routing or SD-WAN control plane must then install an alternative, and only then do applications rebuild their sessions. Annual downtime is that outage multiplied by how often it happens. A 50 ms hardware protection claim means nothing if BFD is set to 300 ms × 3 and TLS sessions take five seconds to re-establish — this shows where the real second is spent.
WAN Failover
detection = BFD interval × multiplier; failover = detection + reconvergence; user outage = failover + application recovery.
detection = BFD interval × multiplier; failover = detection + reconvergence; user outage = failover + application recovery. Failover is a sum of serial delays: the liveness protocol must first declare the path dead, the routing or SD-WAN control plane must then install an alternative, and only then do applications rebuild their sessions. Annual downtime is that outage multiplied by how often it happens.
A 50 ms hardware protection claim means nothing if BFD is set to 300 ms × 3 and TLS sessions take five seconds to re-establish — this shows where the real second is spent.
This calculator takes 5 inputs: BFD transmit interval, BFD detect multiplier, Routing reconvergence, Application session recovery, Failover events per year. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.