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Setting the Values of MSTP Timers
Context
There are three timers used in spanning tree calculation: Forward Delay, Hello Time, and Max Age. These timers can be configured to affect STP convergence. However, you are not advised to directly change these timers. Instead, it is recommended that you set the network diameter so that the spanning tree protocol automatically adjusts these timers in accordance with the network scale.
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 for the new configuration BPDU to spread throughout the entire network. As a result, the original blocked port may be unblocked before a new port is blocked, creating a loop on the network. The purpose of the Forward Delay timer is 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 an interval of Hello Time x 3 x Timer Factor, the device recalculates the spanning tree.
- Max Age: determines whether a BPDU has timed out. A 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.
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)
