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. A path failure can cause spanning tree recalculation 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 require twice the forward delay time before they transit to the forwarding state. This allows the new configuration BPDU to propagate throughout the network.
  • Hello time: The device sends hello packets at the hello time interval to the neighboring devices to make sure the paths are fault-free.
  • 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.