Quiz Entry - updated: 2026.07.14
What is the IEEE 802.1Q VLAN tag, and what are its four fields?
The 802.1Q tag is a 4-byte field inserted into the Ethernet frame header between the source MAC (Media Access Control) address and the Type/Length field, identifying which VLAN (Virtual Local Area Network) the frame belongs to.
The 802.1Q tag inserted into an Ethernet frame. — Bill Stafford, CC BY-SA 3.0, via Wikimedia Commons.
Tag structure (4 bytes total):
| Field | Size | Purpose |
|---|---|---|
| Type (TPID — Tag Protocol Identifier) | 2 bytes | Set to 0x8100 — identifies this as an 802.1Q-tagged frame |
| User Priority (PRI) | 3 bits | CoS (Class of Service) priority level (0–7) for QoS (Quality of Service) — used by voice/video traffic |
| CFI | 1 bit | Canonical Format Indicator — supports Token Ring frames on Ethernet (legacy) |
| VLAN ID (VID) | 12 bits | The actual VLAN number — supports up to 4,096 VLANs (2^12) |
Important behaviors:
- When a frame enters a trunk, the switch inserts the 802.1Q tag and recalculates the FCS (Frame Check Sequence) (because the frame changed)
- When a frame exits to an end device, the switch removes the tag and recalculates the FCS back to the original
- Frames on the native VLAN are sent without a tag on trunk links — this is the key exception
Tip: End devices never see VLAN tags — the switch handles all tagging and untagging transparently.
Go deeper:
IEEE 802.1Q (Wikipedia) — spells out the exact tag layout and all four fields (TPID 0x8100, PCP, DEI, 12-bit VID) and where the tag sits in the frame.