In which STP state does a port not transmit user traffic but listens for BPDUs?

In the Spanning Tree Protocol (STP), the port state where it does not transmit user traffic but continues to listen for Bridge Protocol Data Units (BPDUs) is the blocking state. In this state, the port is effectively disabled for data transmission, which means it won't forward any frames onto the network. However, it remains active to receive and process BPDUs to maintain information about the network topology and to participate in the decision-making process regarding which ports should be forwarding traffic or which should be blocked.

The blocking state is crucial for preventing loops in the network. By blocking certain ports, STP can keep the network stable and avoid broadcast storms that can occur due to loops. If a topology change is detected through received BPDUs, the port may transition out of blocking to a state that permits traffic forwarding.

In contrast, during the learning or forwarding states, ports are actively involved in the transmission of traffic, and while they may listen for BPDUs, they are not restricted as in the blocking state. The listening state specifically refers to a transitional phase prior to the learning state where the port prepares to handle traffic. Thus, the blocking state is the correct choice for a port that listens for BPDUs without forwarding user traffic.