The normal operation of Spanning Tree Protocol (STP) ensures a loop-free topology in Ethernet networks. STP identifies and blocks redundant paths that could cause network loops, allowing only one active path between network devices. It dynamically adjusts to network topology changes by re-calculating the tree and updating the status of ports to maintain network stability and prevent broadcast storms.
STP works step by step by initially electing a root bridge, which is the switch with the lowest bridge ID. All other switches determine their shortest path to the root bridge and assign roles to their ports: root ports (the shortest path to the root bridge), designated ports (the best path to a network segment), and non-designated ports (blocked to prevent loops). If a topology change occurs, STP re-calculates the paths and reassigns port roles to maintain a loop-free network.
The operation of STP involves sending Bridge Protocol Data Units (BPDUs) between switches to share information about network topology. Switches use BPDUs to elect a root bridge and to exchange configuration messages. STP processes these messages to determine the best path to the root bridge and to decide which ports to block or keep active. This ongoing exchange of BPDUs allows STP to adapt to network changes and maintain loop-free operation.
The mechanism of STP includes several states for each port: blocking, listening, learning, and forwarding. In the blocking state, ports do not forward frames to prevent loops. In the listening state, ports process BPDUs to participate in root bridge election and path selection. In the learning state, ports update their MAC address table but do not forward frames. In the forwarding state, ports actively forward frames. These states help STP manage network topology and ensure reliable data transmission without loops.
