Routing And Switching Questions Long
Spanning Tree Protocol (STP) is a network protocol that prevents loops in Ethernet networks by creating a loop-free logical topology. It accomplishes this by selecting a single, designated bridge as the root bridge and then determining the best path from each bridge to the root bridge. This process ensures that there is only one active path between any two network devices, preventing network loops.
The process of STP can be described in the following steps:
1. Bridge Election: Each bridge in the network participates in the bridge election process to determine the root bridge. The bridge with the lowest bridge ID (combination of priority and MAC address) is elected as the root bridge. The root bridge becomes the reference point for all other bridges in the network.
2. Root Bridge Selection: Once the root bridge is elected, all other bridges determine their shortest path to the root bridge. Each bridge calculates its path cost by adding the cost of the incoming port to the cost advertised by the neighboring bridge. The bridge with the lowest path cost to the root bridge becomes the designated bridge for that segment.
3. Port Roles: Each bridge port is assigned a specific role based on its relationship to the root bridge. The root port is the port on each non-root bridge that offers the shortest path to the root bridge. The designated port is the port on each segment that is selected as the best path to reach the root bridge. All other ports are placed in a blocking state, preventing any traffic from passing through them.
4. Topology Change Notification: STP constantly monitors the network for any changes in the topology. When a change is detected, such as a link failure or addition, a topology change notification is sent to all bridges in the network. This triggers the recalculation of the spanning tree and the re-election of the root bridge if necessary.
5. Spanning Tree Recalculation: When a topology change occurs, each bridge recalculates its shortest path to the root bridge. This ensures that the network remains loop-free and that the best path is selected for each bridge.
By following these steps, STP prevents network loops by creating a loop-free logical topology. It ensures that there is only one active path between any two network devices, while other redundant paths are placed in a blocking state. This redundancy allows for failover in case of link failures, ensuring network availability and stability.