The forward delay timer controls how long an STP port remains in the listening and learning states before forwarding frames. Its default is 15 seconds per state, so a newly enabled port takes 30 seconds to forward frames. section 14.4.2
Use these timing points to understand STP convergence:
- Listening lets STP determine the port role; learning lets the switch build its MAC address table. section 14.6
- The timer helps prevent loops by delaying a transition into forwarding. section 14.4.2
- After a topology change, STP can take up to 50 seconds: 20 seconds of max age plus 30 seconds of forward delay. 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 forward delay timer with the hello and max age timers, including their purposes and default durations. section 14.4.2
