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STP - Port state from Listening to Learning

 When Does a Switch Transition from Listening to Learning State, and Can It Revert to Blocking?


A switch moves a port from the listening state to the learning state under the following condition:

Transition from Listening to Learning:

  • Successful BPDU Processing: If the port in the listening state continues to receive BPDUs and determines that the path through this port can be part of the active spanning tree, it transitions to the learning state. In this state, the port starts learning MAC addresses but still does not forward any data frames.

At this stage, the switch is confident that the port may become part of the forwarding path, but it has not yet fully committed to forwarding traffic. Instead, it prepares by populating its MAC address table with the source addresses of the incoming frames to optimize future forwarding decisions.

Scenario: Switch Returning from Listening to Blocking

Yes, there are scenarios where a switch can transition from the listening state back to the blocking state:

  1. Receiving a Superior BPDU from Another Port: While in the listening state, if the switch receives a BPDU from another port that indicates a better path to the root bridge (lower bridge ID or path cost), the switch may decide that the current port is no longer the best candidate to forward traffic. In this case, the port will revert to the blocking state.

  2. Topology Change: If a topology change occurs during the listening state (e.g., a new root bridge is elected, or a better path to the root bridge is found), the switch might re-evaluate the role of the port. If the re-evaluation determines that the port is not needed for forwarding, it transitions back to the blocking state.

In both scenarios, the switch dynamically reconfigures its spanning tree structure to ensure that no loops occur in the network and that the most optimal path to the root bridge is maintained.

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