802.1Q
acronymStands for: IEEE 802.1Q (VLAN tagging)
The IEEE standard that adds a VLAN tag to an Ethernet frame - the reason one physical link can carry many virtual LANs.
802.1Q inserts a four-byte tag carrying a 12-bit VLAN ID (4,094 usable) and a priority field into the Ethernet header. A tagged link between switches is a trunk; untagged access ports drop a device into a single VLAN. It is the foundation of segmentation on switched networks, and it lives in the 802.1 working group, the architecture arm of the 802 family.
The tag is four octets inserted after the source address: the identifier 0x8100, which IANA lists as the Customer VLAN Tag Type, then three bits of priority (the PCP), one bit marking the frame drop-eligible (the DEI, once called CFI), and the 12-bit VLAN identifier. Of its 4,096 values two are kept out: 0 marks a frame that carries only a priority, and 4095 is reserved, which leaves VLANs 1 to 4094.
The standard has grown far past the tag. IEEE 802.1Q-2022, the active edition, specifies the operation of MAC Bridges and VLAN Bridges and has absorbed 802.1D. Its own vocabulary is not the vendors': a port has a member set and an untagged set, and a PVID, the VLAN an untagged frame is placed in, 1 by default.
Provider networks stack a second tag in front of the first, with its own identifier, 0x88A8, introduced by 802.1ad in 2005 and since folded into 802.1Q.
Also known as: 802.1q, dot1q, vlan tag, vlan trunking