Failover Time Calculator
Work out failover time from BFD or hello timers plus switchover, and turn it into packet loss and yearly downtime.
About Failover Time Calculator
Detection normally dominates, which is why BFD exists: it runs a lightweight session at millisecond intervals instead of relying on protocol hellos measured in seconds. Link-down detection is faster still, but only works when the failure is a local physical one. Availability targets are usually written as nines, and this converts a timer setting into that language — 6 events at a second each is well inside five nines, while 6 events at 40 seconds each is not.
Formulas
Failover Time
Failover = detection (interval × multiplier, or ~10 ms for link-down) + control-plane switchover + FIB update.
- interval × multiplier
- BFD or hello detection time — 300 ms × 3 is a common conservative setting
- switchover
- Time for the control plane to select and signal the backup path
- FIB update
- Time to reprogram hardware forwarding entries
Availability impact
downtime per year = failover time × events; availability = 100 − downtime ÷ 31,536,000,000 ms.
Worked example
With the default values below, Failover Time Calculator returns Total Failover Time of 1,050.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Detection mechanismone of BFD, IGP hellos, BGP keepalives, Physical link down
- BFD
- Detection intervalminimum 1 ms
- 300 ms
- Detection multiplieraccepts 1–50
- 3
- Switchover timeminimum 0 ms
- 50 ms
- FIB update timeminimum 0 ms
- 100 ms
- Packet rate through the pathminimum 0 pps
- 50,000 pps
- Failover events per yearaccepts 0–1,000 events
- 6 events
Results
- Total Failover Time
- 1,050.0 ms
- Detection Time
- 900.0 ms
- Packets Lost per Event
- 52,500
- Downtime per Year
- 6.30 s
- Availability from Failovers Alone
- 99.999980%
- Assessment
- Poor — over a second, sessions and real-time media will drop
Step by step
- 1Values used
- 2Failover Time
- 3Availability impact
- 4Total Failover Time= 1,050.0 ms
- 5Detection Time= 900.0 ms
- 6Packets Lost per Event= 52,500
- 7Downtime per Year= 6.30 s
- 8Availability from Failovers Alone= 99.999980
- 9Assessment= Poor — over a second, sessions and real-time media will drop
Assumptions and limits
- The result is derived only from the 7 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 6 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 Failover Time calculated?
Failover = detection (interval × multiplier, or ~10 ms for link-down) + control-plane switchover + FIB update. Detection normally dominates, which is why BFD exists: it runs a lightweight session at millisecond intervals instead of relying on protocol hellos measured in seconds. Link-down detection is faster still, but only works when the failure is a local physical one.
Why does Failover Time matter?
Availability targets are usually written as nines, and this converts a timer setting into that language — 6 events at a second each is well inside five nines, while 6 events at 40 seconds each is not.
What values do I need to enter?
This calculator takes 7 inputs: Detection mechanism, Detection interval, Detection multiplier, Switchover time, FIB update time, Packet rate through the path, 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.
Why not set BFD to 50 ms × 3 everywhere?
Aggressive timers put load on the platform and risk false positives when the control plane is busy — a single missed window tears down a working path. Verify the platform does BFD in hardware or a dedicated process before going below 300 ms, and never tune it on a software-forwarding device.