BPDU Timer Calculator
Validate STP hello, max-age and forward-delay timers against the IEEE inequalities and get the convergence time.
Inputs
Convergence Time
6.0s
Classic STP Convergence
50.0s
BPDUs Missed Before Failure Declared
10
Bridge Diameter Supported
8bridges
Timers Satisfy IEEE Rules
Yes — both IEEE inequalities are satisfied
Step by step
Values used
Hello time = 2 s; Max age = 20 s; Forward delay = 15 s; Spanning-tree version = Rapid STP — 802.1w
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.
802.1w convergence
Rapid STP converges in roughly 3 × hello, because it uses proposal/agreement instead of timers.
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
How it works
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.
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.