In STP, how long does a port typically remain in Blocking mode before transitioning?

In Spanning Tree Protocol (STP), a port typically spends a period in Blocking mode before it can transition to another state, which is necessary to ensure that there are no loops within the network topology. The standard time frame for a port to remain in the Blocking state is determined by the Bridge Protocol Data Unit (BPDU) timers, specifically the Forward Delay timer.

When STP is initiated, ports begin in the Blocking state to prevent any loops while the switches exchange BPDUs to determine the best path through the network. After the listening and learning states, and depending on the network’s topology, a port in Blocking mode will generally take about 20 seconds of listening and 15 seconds of learning to reach the Forwarding state. However, it is important to note that the exact duration a port remains in Blocking mode is influenced by the Forward Delay timer which is commonly set to 15 seconds by default on Cisco switches.

Thus, while the entire process may take longer, blocking remains a critical state as the protocol ensures network stability and prevents loops before allowing any forwarding of traffic. The choice of 20 seconds correctly reflects the time a port spends in Blocking mode within the overall transition process in STP, aligning with standard configurations and behavior observed in