Final answer:
The term that describes a field used to specify a VLAN ID in a switch running STP is Port ID.
Step-by-step explanation:
The term that describes a field used to specify a VLAN ID in a switch running STP is Port ID. The Port ID is a 12-bit value that includes a VLAN ID field. This field is used to identify the VLAN associated with a particular port on the switch. For example, if a switch has multiple VLANs configured and STP is enabled, the Port ID field will indicate the VLAN ID for each port.
In the context of STP, the Extended system ID is a component of the Bridge Protocol Data Unit (BPDU) which is vital for the operation of STP. It extends the original 16-bit bridge priority field to include the VLAN ID, allowing STP to operate on a per-VLAN basis, which is known as Per-VLAN Spanning Tree (PVST) or Multiple Spanning Tree (MST).
The Extended system ID is used alongside the bridge priority and the switch's MAC address to form the bridge ID (BID), which determines the root bridge in an STP topology. The choice of root bridge is critical because it serves as the reference point for all path calculations within a network.