What determines how often a nonroot bridge sends an 802.1D STP Hello BPDU message?

The frequency with which a non-root bridge sends an 802.1D Spanning Tree Protocol (STP) Hello Bridge Protocol Data Unit (BPDU) message is determined by the Hello timer configured on the root switch. This timer controls the intervals at which BPDUs are transmitted to ensure the topology remains stable and loops are avoided.

In a typical STP environment, the root bridge sends BPDUs at regular intervals, and non-root bridges must adhere to this timeframe. Essentially, non-root bridges listen to the BPDUs from the root bridge to learn about the network topology and must send their own BPDUs based on the timing set by the root. Therefore, the correct answer underscores the hierarchical nature of STP, where the root's configuration governs the rest of the network's behavior.

The other choices do not influence this specific timing mechanism. For instance, port cost relates to the selection of the best path but does not determine how often BPDUs are sent. Similarly, the status of a root port affects data forwarding but not BPDU transmission intervals. The bridge port ID identifies the port on which the BPDU is being sent, but again, it does not have a bearing on the Hello timer settings.