For RSTP to rapidly recover network connectivity, RSTP requires timer expiration. RSTP derives many of the timer values from the existing configured STP (Spanning Tree Protocol) timers to meet its rapid recovery requirements rather than relying on additional timer configurations.
User-Configurable Timers describes the user-configurable timers, and Derived Timers describes the timers that are derived from other timers and are not user configurable.
User-Configurable Timers
| Timer | Description |
|---|---|
| Hello | The root bridge uses the hello timer to send out configuration BPDUs through all of its forwarding ports at a predetermined, regular time interval. The default value is 2 seconds. The range is 1 to 10 seconds. |
| Forward delay | A port moving from the blocking state to the forwarding state uses the forward delay timer to transition through the listening and learning states. In RSTP, this timer complements the rapid configuration behavior. If none of the rapid rules are in effect, the port uses legacy STP rules to move to the forwarding state. The default is 15 seconds. The range is 4 to 30 seconds. |
Derived Timers
| Timer | Description |
|---|---|
| TCN | The root port uses the topology change notification (TCN) timer when it detects a change in the network topology. The TCN timer stops when the topology change timer expires or upon receipt of a topology change acknowledgement. The default value is the same as the value for the bridge hello timer. |
| Topology change | The topology change timer
determines the total time it takes the forwarding ports to send
configuration BPDUs. The default value for the topology change timer
depends upon the mode of the port: 802.1D mode—The sum of the forward delay timer value (default value is 15 seconds; range of 4 to 30 seconds) and the maximum age timer value (default value is 20 seconds; range of 6 to 40 seconds). 802.1w mode—Double the hello timer value (default value is 4 seconds). |
| Message age | A port uses the message age timer to time out receiving BPDUs. When a port receives a superior or equal BPDU, the timer restarts. When the timer expires, the port becomes a designated port and a configuration update occurs. If the bridge operates in 1w mode and receives an inferior BPDU, the timer expires early. The default value is the same as the STPD (Spanning Tree Domain) bridge max age parameter. |
| Hold | A port uses the hold timer to restrict the rate that successive BPDUs can be sent. The default value is the same as the value for the bridge hello timer. |
| Recent backup | The timer starts when a port leaves the backup role. When this timer is running, the port cannot become a root port. The default value is double the hello time (4 seconds). |
| Recent root | The timer starts when a port leaves the root port role. When this timer is running, another port cannot become a root port unless the associated port is put into the blocking state. The default value is the same as the forward delay time. |
The protocol migration timer is neither user-configurable nor derived; it has a set value of 3 seconds. The timer starts when a port transitions from STP (802.1D) mode to RSTP (802.1w) mode and vice-versa. This timer must expire before further mode transitions can occur.
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