STP timers

The most important timing parameters in STP calculation are forward delay, hello time, and max age.

• Forward delay

  Forward delay is the delay time for port state transition. By default, the forward delay is 15 seconds.

  A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to the change. However, the resulting new configuration BPDU cannot propagate throughout the network immediately. If the newly elected root ports and designated ports start to forward data immediately, a temporary loop will likely occur.

  The newly elected root ports or designated ports must go through the listening and learning states before they transit to the forwarding state. This requires twice the forward delay time and allows the new configuration BPDU to propagate throughout the network.

• Hello time

  The device sends configuration BPDUs at the hello time interval to the neighboring devices to ensure that the paths are fault-free. By default, the hello time is 2 seconds. 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

  The device uses the max age to determine whether a stored configuration BPDU has expired and discards it if the max age is exceeded. By default, the max age is 20 seconds. 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.

If a port does not receive any configuration BPDUs within the timeout period, the port transits to the listening state. The device will recalculate the spanning tree. It takes the port 50 seconds to transit back to the forwarding state. This period includes 20 seconds for the max age, 15 seconds for the listening state, and 15 seconds for the learning state.

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)