Which one of these fields contributes to the uniqueness of a bridge ID?

The uniqueness of a bridge ID is primarily determined by the MAC address of the bridge itself. The bridge ID is a combination of the bridge priority and the MAC address, which is designed to ensure that each bridge on a network can be uniquely identified. While the bridge priority is an important factor in determining which bridge will be the root bridge in a Spanning Tree Protocol (STP) topology, it does not contribute to the uniqueness of the bridge ID on its own.

The VLAN ID is relevant in the context of extending the bridge ID for multiple VLANs, particularly in situations involving multiple spanning tree instances, but it does not contribute directly to the uniqueness of a single bridge's ID. Extended System ID is an addition that helps identify the VLAN number under which a bridge operates but also doesn't create uniqueness by itself. Thus, the MAC address is essential as it allows each bridge to be distinguished from others in the network, contributing significantly to the overall uniqueness of the bridge ID.