Failover Time Calculator
Work out failover time from BFD or hello timers plus switchover, and turn it into packet loss and yearly downtime.
Inputs
Total Failover Time
1,050.0ms
Detection Time
900.0ms
Packets Lost per Event
52,500
Downtime per Year
6.30s
Availability from Failovers Alone
99.999980%
Assessment
Poor — over a second, sessions and real-time media will drop
Step by step
Values used
Detection mechanism = BFD; Detection interval = 300 ms; Detection multiplier = 3; Switchover time = 50 ms; FIB update time = 100 ms; Packet rate through the path = 50,000 pps; Failover events per year = 6 events
Failover Time
Failover = detection (interval × multiplier, or ~10 ms for link-down) + control-plane switchover + FIB update.
Availability impact
downtime per year = failover time × events; availability = 100 − downtime ÷ 31,536,000,000 ms.
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
How it works
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.
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.