On a trunk link, which field identifies the VLAN membership of a frame?

• A. MAC address
• B. VLAN Tag
• C. IP address
• D. Frame check sequence

Answer: (B)

On a trunk link, frames from multiple VLANs share the same physical connection, so the network must know which VLAN each frame belongs to. That identification is carried by the VLAN tag in the Ethernet frame. The VLAN tag, created by 802.1Q tagging, includes the VLAN ID, which tells switches exactly which VLAN the frame belongs to as it’s forwarded across the trunk. The other fields don’t indicate VLAN membership: the MAC address is just the device’s hardware address, the IP address is a layer-3 address used for routing, and the Frame Check Sequence is used for error detection. So the VLAN tag is the precise field that identifies VLAN membership on a trunk.