VLAN Tag Length in Ethernet Frames | Generated by AI
Question: In a Virtual Local Area Network (VLAN) environment, what is the length of the VLAN tag field within an Ethernet frame format (in bytes)?
Answer: In a Virtual Local Area Network (VLAN) environment, the length of the VLAN tag field is 4 bytes (or 32 bits).
Breakdown of the 802.1Q VLAN Tag
When a standard Ethernet frame is converted into a tagged frame, a 4-byte header is inserted between the Source Address and the Ethertype/Length fields. This standard is defined by IEEE 802.1Q.
The 4-byte (32-bit) tag is further divided into specific sub-fields:
- TPID (Tag Protocol Identifier) - 2 Bytes:
- Set to a fixed value of
0x8100. - This identifies the frame as an IEEE 802.1Q tagged frame.
- Set to a fixed value of
- TCI (Tag Control Information) - 2 Bytes: This section is subdivided into:
- PCP (Priority Code Point): 3 bits used for Quality of Service (QoS) to prioritize traffic.
- DEI (Drop Eligible Indicator): 1 bit indicating if the frame can be dropped during congestion.
- VID (VLAN Identifier): 12 bits used to identify the specific VLAN (supporting up to 4,096 unique IDs).
Key Takeaway
While a standard Ethernet frame header is 14 bytes, adding the 4-byte VLAN tag increases the header to 18 bytes. This is why the Maximum Transmission Unit (MTU) or the maximum frame size often needs to be adjusted in networking equipment to account for this overhead.
Would you like me to explain how the 12-bit VLAN ID limits the total number of VLANs you can create?