STP Port States:- Blocking, Listing, Learning, and Forwarding

STP (Spanning Tree Protocol) is a layer two protocol. It finds and removes the switching loops from the network. A switching loop occurs when two switches have more than one direct connection between them. To prevent loops in the network, STP assigns ports to four states: blocking, listening, learning, and forwarding. During these states, the STP running switch learns the network topology, selects the root and designated ports, and disables the remaining ports to remove loops.

STP Blocking state

When you start an STP running switch, it does not enable its ports immediately. It keeps all ports in the blocking state for the first 20 seconds. In this period, it accepts and processes only BPDU messages. A BPDU contains the switch's priority ID, MAC address, and other necessary information. Switches use this information to elect the root bridge. Besides the BPDUs, it drops all other frames. From incoming BPDUs, it learns the network topology and determines which ports will function as root, designated, and blocked ports. After twenty seconds, only the root port and designated ports move into the next state. The remaining ports stay in this state.

STP Listening state

In this state, the switch double-checks the network topology to confirm that the ports it selected as root and designated ports do not form a loop. Ports remain in this state for the next fifteen seconds. During this state, ports listen and process only BPDUs. They do not process the user frames.

STP Learning state

In this state, ports start processing user frames. They use incoming frames to build the CAM table. However, they do not forward the incoming frames. Ports remain in this for another fifteen seconds.

STP Forwarding state

It is the final state of the STP operation. It is also called convergency. Only the root and designated ports reach this state. STP disables all remaining ports. In this state, ports listen and process both BPDUs and user frames. It uses BPDUs to monitor the network.

If any change occurs in the network, it repeats the same process on the affected ports.

STP Disabled State

It is a unique port state. STP uses it for all ports that do not participate in STP. Many reasons could prevent a port from participating in the STP operation. For example, an administrator may manually remove a port from STP operation for security reasons, the cable connected to the port may be unplugged, or the port may have no connection. In this case, STP does not send or receive any BPDUs.

Conclusion

STP is essential for maintaining a loop-free and reliable network. By systematically transitioning ports through various states and dynamically responding to network changes, STP ensures optimal data flow and network stability.