BPDU Timer Calculator
Validate STP hello, max-age and forward-delay timers against the IEEE inequalities and get the convergence time.
About BPDU Timer Calculator
Classic STP is timer-driven: a bridge waits max age to notice the root is gone, then 15 seconds each in listening and learning, giving the familiar 50-second outage. RSTP replaces the wait with an explicit handshake on point-to-point links, so hello timers only matter for detecting a dead neighbour. Tuning timers down to speed up convergence is how people accidentally build an unstable domain — below the IEEE minimums a BPDU can expire in transit and the topology never settles.
Formulas
BPDU Timer
IEEE 802.1D requires max age ≥ 2 × (hello + 1) and forward delay ≥ (max age ÷ 2) + 1; classic convergence = max age + 2 × forward delay.
- hello time
- BPDU transmission interval, default 2 s
- max age
- Age at which a stored BPDU is discarded, default 20 s
- forward delay
- Time spent in each of listening and learning, default 15 s
802.1w convergence
Rapid STP converges in roughly 3 × hello, because it uses proposal/agreement instead of timers.
Worked example
With the default values below, BPDU Timer Calculator returns Convergence Time of 6.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Hello timeaccepts 1–10 s
- 2 s
- Max ageaccepts 6–40 s
- 20 s
- Forward delayaccepts 4–30 s
- 15 s
- Spanning-tree versionone of Classic STP — 802.1D, Rapid STP — 802.1w
- Rapid STP — 802.1w
Results
- Convergence Time
- 6.0 s
- Classic STP Convergence
- 50.0 s
- BPDUs Missed Before Failure Declared
- 10
- Bridge Diameter Supported
- 8 bridges
- Timers Satisfy IEEE Rules
- Yes — both IEEE inequalities are satisfied
Step by step
- 1Values used
- 2BPDU Timer
- 3802.1w convergence
- 4Convergence Time= 6.0 s
- 5Classic STP Convergence= 50.0 s
- 6BPDUs Missed Before Failure Declared= 10
- 7Bridge Diameter Supported= 8 bridges
- 8Timers Satisfy IEEE Rules= Yes — both IEEE inequalities are satisfied
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, 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 BPDU Timer calculated?
IEEE 802.1D requires max age ≥ 2 × (hello + 1) and forward delay ≥ (max age ÷ 2) + 1; classic convergence = max age + 2 × forward delay. Classic STP is timer-driven: a bridge waits max age to notice the root is gone, then 15 seconds each in listening and learning, giving the familiar 50-second outage. RSTP replaces the wait with an explicit handshake on point-to-point links, so hello timers only matter for detecting a dead neighbour.
Why does BPDU Timer matter?
Tuning timers down to speed up convergence is how people accidentally build an unstable domain — below the IEEE minimums a BPDU can expire in transit and the topology never settles.
What values do I need to enter?
This calculator takes 4 inputs: Hello time, Max age, Forward delay, Spanning-tree version. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Should I lower the timers to converge faster?
No — move to RSTP or MSTP instead. Timer tuning trades stability for speed and only pays off in a small, well-understood diameter; RSTP gets you sub-second convergence without breaking the inequalities.