If a device does not receive any BPDUs from the upstream device within the timeout interval, the device considers the upstream device to have failed and recalculates the spanning tree.
Sometimes, a device cannot receive the BPDU from the upstream device within the timeout interval because the upstream device is busy. In this case, recalculating the spanning tree will cause a waste of network resources. To avoid wasting network resources, set a long timeout interval on a stable network.
If a device does not receive any BPDUs from the upstream device within the timeout interval, spanning tree recalculation is performed. The timeout interval is calculated as follows:
Timeout interval = Hello time x 3 x Timer Factor