What does the Tag Control Identifier (TCI) carry?

The Tag Control Identifier (TCI) is a crucial component of the IEEE 802.1Q standard used in networking, particularly in VLAN (Virtual Local Area Network) tagging. The TCI includes the VLAN ID, which is a unique identifier for each VLAN within a network. This ID allows switches and routers to distinguish between different VLANs on the same physical network infrastructure.

By using the VLAN ID carried via the TCI, network traffic can be properly segmented, allowing for efficient communication and management of different network segments. This is important for maintaining network security and performance, as it helps ensure that data packets are delivered to the correct logical network.

While other identifiers like data type, packet priority, and stream identifiers are prevalent in networking, they do not represent what the TCI carries. Specifically, packet priority is addressed in a separate field within VLAN tagging (the Priority Code Point), but it is not the primary function of the TCI itself. Instead, the fundamental purpose of the TCI is to convey the VLAN ID, which is essential for identifying the virtual networks associated with the packets.