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Setting RSTP Timers

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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. When this occurs, a loop exists 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 switching device sends configuration BPDUs at the specified interval to detect link failures. If the switching device does not receive any BPDUs within an interval of Hello Time, the switching device recalculates the spanning tree.
Max Age: determines whether 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 of Forward Delay, Hello Time, and Max Age.

You are not advised to directly change the preceding three timers. The three parameters are relevant 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 retain 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 - 1.0 second) >= Max Age

Max Age >= 2 x (Hello Time + 1.0 second)

The system view is displayed.

Set the Forward Delay, Hello Time, and Max Age timers.
stp timer forward-delay forward-delay
The Forward Delay timer is set for the switching device.

By default, the Forward Delay timer is 1500 centiseconds (15 seconds).

stp timer hello hello-time
The Hello Time is set for the switching device.

By default, the Hello Time is 200 centiseconds (2 seconds).

stp timer max-age max-age
The Max Age timer is set for the switching device.

By default, the Max Age timer is 2000 centiseconds (20 seconds).
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