IEEE 802.1Q VLAN Tag Explained | Generated by AI
Question: According to the IEEE 802.1Q standard, which defines the Ethernet frame format in a virtual LAN environment, what is the 4-byte field used to indicate which virtual LAN the host sending the frame belongs to?
Answer:
The correct answer is VLAN Tag (also commonly called VLAN标记 or simply Tag in Chinese technical materials).
Explanation:
IEEE 802.1Q is the industry standard for implementing VLANs (Virtual Local Area Networks) on Ethernet networks. It defines how to insert an additional 4-byte field into the standard Ethernet frame to carry VLAN information.
This 4-byte field is officially named the Tag in the 802.1Q standard, and it is most commonly referred to in networking as the VLAN Tag.
The structure of the 802.1Q-tagged Ethernet frame is:
- Destination MAC Address (6 bytes)
- Source MAC Address (6 bytes)
- Tag Protocol Identifier (TPID) — 2 bytes (always 0x8100 for 802.1Q)
- Tag Control Information (TCI) — 2 bytes, which contains:
- Priority Code Point (PCP) — 3 bits
- Drop Eligible Indicator (DEI) — 1 bit (formerly CFI)
- VLAN Identifier (VID) — 12 bits (this is the actual VLAN number, 0–4095)
- EtherType / Length (2 bytes)
- Payload
- Frame Check Sequence (FCS)
The complete 4-byte tag header (TPID + TCI) is what identifies the VLAN membership of the frame. Therefore, the field being asked for in the question is the VLAN Tag (or simply Tag).
In most textbooks, certification materials (CCNA, HCIA, etc.), and exam questions in Chinese, this 4-byte field is directly referred to as VLAN标记 or Tag.
So the reference answer “VLAN标记/Tag” is correct.
References:
- None (standard networking knowledge based on IEEE 802.1Q specification)