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WAN Failover Calculator

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

About WAN Failover Calculator

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

Worked example

With the default values below, WAN Failover Calculator returns Network Failover Time of 1,100. Change any input above and every figure here is recalculated in your browser.

Inputs used

BFD transmit intervalaccepts 3–5,000 ms
300 ms
BFD detect multiplieraccepts 1–20 x
3 x
Routing reconvergenceaccepts 0–60,000 ms
200 ms
Application session recoveryaccepts 0–600 s
5 s
Failover events per yearaccepts 0–500 events
4 events

Results

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%

Step by step

  1. 1Values used
  2. 2WAN Failover
  3. 3Network Failover Time= 1,100 ms
  4. 4BFD Detection Time= 900 ms
  5. 5User-visible Outage= 6.10 s
  6. 6Annual Downtime= 0.407 minutes
  7. 7Resulting Availability= 99.99992

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

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.

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WAN Failover Calculator – Calculate Network Failover Time