Setting spanning tree timers
The following timers are used for spanning tree calculation:
Forward delay—Delay time for port state transition. To prevent temporary loops on a network, the spanning tree feature sets an intermediate port state (the learning state) before it transits from the discarding state to the forwarding state. The feature also requires that the port transit its state after a forward delay timer. This ensures that the state transition of the local port stays synchronized with the peer.
Hello time—Interval at which the device sends configuration BPDUs to detect link failures. If the device does not receive configuration BPDUs within the timeout period, it recalculates the spanning tree. The formula for calculating the timeout period is timeout period = timeout factor × 3 × hello time.
Max age—In the CIST of an MSTP network, the device uses the max age timer to determine whether a configuration BPDU received by a port has expired. If it is expired, a new spanning tree calculation process starts. The max age timer does not take effect on MSTIs.
To ensure a fast topology convergence, make sure the timer settings meet the following formulas:
2 × (forward delay – 1 second) ≥ max age
Max age ≥ 2 × (hello time + 1 second)
Do not manually set the spanning tree timers. As a best practice, specify the network diameter and letting spanning tree protocols automatically calculate the timers based on the network diameter. If the network diameter uses the default value, the timers also use their default values.
Set the timers only on the root bridge. The timer settings on the root bridge apply to all devices on the entire switched network.