VLANTags

VLANTags are a mechanism used in Ethernet networks to classify frames by VLAN, enabling traffic to be carried over shared physical links while remaining logically separated into broadcast domains. They are essential for network segmentation, quality of service, and multi-tenant deployments.

The tagging standard is IEEE 802.1Q. A tag is a four-byte field inserted into the Ethernet frame between the source MAC address and the EtherType/length field. The tag starts with a two-byte Tag Protocol Identifier (TPID) value of 0x8100, followed by a two-byte Tag Control Information (TCI). The TCI contains a three-bit Priority Code Point (PCP), a one-bit Drop Eligible Indicator (DEI, formerly called CFI), and a twelve-bit VLAN Identifier (VID). The VID identifies the VLAN; valid IDs range from 1 to 4094. A VID of 0 is used for priority tagging (no VLAN) and 4095 is reserved for future use.

On access ports, frames are typically untagged and associated with a single VLAN. On trunk ports, traffic from multiple VLANs is carried by tagging frames with the appropriate VID. The native VLAN on a trunk is usually sent untagged; mismatches between configured native VLANs on connected devices can lead to security and forwarding issues.

History and variants: 802.1Q was standardized in 1998 and is widely supported across Ethernet devices. Inter-switch Link (ISL) is an older Cisco-proprietary tagging scheme that has largely fallen out of use. Some networks employ QinQ (802.1ad) to nest VLAN tags for service-provider or data-center scenarios, enabling additional layering of VLANs.

VLANTags underpin modern network design by enabling scalable segmentation, policy enforcement, and efficient use of physical infrastructure.