On a network running VBST, when the network topology becomes stable, the non-root-bridge switch forwards BPDUs sent by the root bridge to neighboring switches at an interval of Hello time to check whether links are faulty. If the switch does not receive any BPDU from the upstream device within the timeout interval, the switch considers that the upstream device fails and recalculates the spanning tree.
Sometimes, the switch may not receive BPDUs in a long time from the upstream device because the upstream device is very busy. In this case, the device should not recalculate its spanning tree. Therefore, you can set a long timeout interval for the device on a stable network to reduce waste of network resources.