What is added to an Ethernet frame by the 802.1Q protocol to specify VLAN membership?

The Tag Protocol Identifier (TPID) is a crucial component added to an Ethernet frame when using the 802.1Q protocol to specify VLAN membership. This identifier helps distinguish between regular Ethernet frames and VLAN-tagged frames. Specifically, the TPID is included in the frame header, and it is set to a specific value, typically 0x8100, which indicates that the following fields of the frame will contain VLAN tagging information.

This tagging mechanism allows the network to effectively handle traffic segmentation and manage network resources by grouping devices into different virtual LANs (VLANs) without requiring separate physical networks. When a switch receives a VLAN-tagged frame, it can determine which VLAN the traffic belongs to and forward it accordingly. This capability is essential for maintaining efficient communications in complex networks where devices may need to share resources but remain logically separate.

The other options do not pertain to the VLAN tagging process defined by 802.1Q. MAC addresses are used for unique device identification on a network, data encryption keys relate to securing communications, and network interface identifiers do not serve a role in specifying VLAN membership within the context of Ethernet frames.