Which field completes the STP Bridge ID along with Bridge Priority and Extended System ID?

The correct choice is the MAC Address, as it plays a crucial role in forming the Bridge ID in the Spanning Tree Protocol (STP). The Bridge ID is a unique identifier that helps in determining the root bridge in a network of switches.

The Bridge ID is composed of two main components: the Bridge Priority and the Extended System ID, which incorporates the VLAN ID. The MAC Address is then appended to this to ensure that each switch within the network has a unique Bridge ID. This uniqueness is essential for STP to effectively prevent loops in network topologies by deciding which switch will become the root bridge based on the lowest Bridge ID.

By including the MAC Address, STP ensures that even when multiple switches have the same Bridge Priority and Extended System ID (which can occur in larger networks), the overall Bridge ID remains unique due to the distinct MAC addresses of each switch. This is vital for maintaining an efficient and loop-free network environment.

Understanding the components of the Bridge ID is important for networking professionals, as it directly relates to how STP functions to maintain network stability.