WAN Failover Calculator

Add up BFD detection, routing convergence and application recovery to get real WAN failover time and downtime.

Inputs

BFD transmit intervalms

BFD detect multiplierx

Routing reconvergencems

Application session recoverys

Failover events per yearevents

Network Failover Time

1,100ms

BFD Detection Time

900ms

User-visible Outage

6.10s

Annual Downtime

0.407minutes

Resulting Availability

99.99992%

Step by step

Values used
BFD transmit interval = 300 ms; BFD detect multiplier = 3 x; Routing reconvergence = 200 ms; Application session recovery = 5 s; Failover events per year = 4 events
WAN Failover
detection = BFD interval × multiplier; failover = detection + reconvergence; user outage = failover + application recovery.
Network Failover Time
= 1,100 ms
BFD Detection Time
= 900 ms
User-visible Outage
= 6.10 s
Annual Downtime
= 0.407 minutes
Resulting Availability
= 99.99992

How it works

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.

Formula

WAN Failover

detection = BFD interval × multiplier; failover = detection + reconvergence; user outage = failover + application recovery.

BFD interval: How often hellos are sent on the circuit
multiplier: Missed hellos tolerated before the session is declared down

Frequently Asked Questions

How is WAN Failover calculated?

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.

Why does WAN Failover matter?

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.

What values do I need to enter?

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.

You might also need

Multi-WAN Capacity Calculator Network Redundancy Calculator SD-WAN Path Selection Calculator