The max age timer is an STP parameter that controls how long a switch waits after it stops receiving BPDUs before recalculating the spanning-tree topology. Its default value is 20 seconds. section 14.4.2
Key points:
- With a 2-second hello timer, a switch can miss 10 BPDUs before starting a recalculation. section 14.4.2
- The switch may elect a new root bridge and recalculate port roles after the timer expires. section 14.4.2
- The max age timer combines with the listening and learning states, producing up to 50 seconds before forwarding resumes. section 14.4.2
Hardware failure
If SW1 stops receiving BPDUs through its root port, that port can remain the root port for 20 seconds. A replacement port then spends 15 seconds listening and 15 seconds learning before forwarding, creating up to 50 seconds of downtime. If the failed port goes down/down, STP skips the max age wait, but the replacement still requires 30 seconds. section 14.4.2
Table 14.3 STP timers (view table figure)
| Timer |
Purpose |
Duration |
|---|---|---|
| Hello |
How often BPDUs are sent |
2 seconds |
| Forward delay |
The length of the listening and learning states |
15 seconds (per state) |
| Max age |
How long a switch will wait to change the STP topology after ceasing to receive BPDUs on a port |
20 seconds |
Compare the hello, forward delay, and max age timers in one reference table. section 14.4.2
