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Setting RSTP Timers
Context
The following timers are used in spanning tree calculation:
- Forward Delay: specifies the delay before a state transition. After the topology of a ring network changes, it takes some time to spread the new configuration BPDU throughout the entire network. As a result, the original blocked port may be unblocked before a new port is blocked. This creates a loop on the network. You can set the Forward Delay timer to prevent loops. When the topology changes, all ports will be temporarily blocked during the Forward Delay.
- Hello Time: specifies the interval at which hello packets are sent. A device sends configuration BPDUs at the specified interval to detect link failures. If the switching device does not receive any BPDUs within the timeout period (timeout period = Hello Time x 3 x Timer Factor), the device recalculates the spanning tree.
- Max Age: determines when BPDUs expire. A switching device determines that a received configuration BPDU times out when the Max Age expires.
Devices on a ring network must use the same values for Forward Delay, Hello Time, and Max Age.
You are not advised to directly change the preceding three parameters as they are related to the network scale; therefore, it is recommended that you set the network diameter so that the spanning tree protocol automatically adjusts these timers. When the default network diameter is used, the three timers also use their default values.
To prevent frequent network flapping, make sure that the Hello Time, Forward Delay, and Max Age timer values conform to the following formulas:
2 x (Forward Delay - 1 second) >= Max Age
Max Age >= 2 x (Hello Time + 1 second)