Which IEEE standard defines VLAN tagging on trunk links and adds a 4-byte tag to Ethernet frames?

Which IEEE standard defines VLAN tagging on trunk links and adds a 4-byte tag to Ethernet frames?

• A. 802.1Q
• B. 802.1X
• C. 802.3
• D. 802.11

Answer: (A)

The main idea being tested is how VLANs are carried across trunk links and how that tagging is implemented at Layer 2. The standard that defines VLAN tagging on trunk links and adds a 4-byte tag to Ethernet frames is IEEE 802.1Q. It inserts a VLAN tag into every frame that travels a trunk, allowing a single physical link to carry traffic for multiple VLANs. The tag sits between the source MAC address and the EtherType/length field, so the frame becomes 4 bytes longer. Inside that tag is the Tag Control Information, which includes the VLAN ID (12 bits) and priority bits (for traffic prioritization), plus a bit for drop eligibility. Frames carrying this tag are identified as VLAN-tagged by the Ethertype value 0x8100, after which the original EtherType/length field appears again. This mechanism is what enables switches to keep traffic segregated by VLAN as it traverses trunk links, while still using a single physical connection. Other options cover different areas: 802.1X is about authenticating devices on a port, 802.3 is the base Ethernet standard for framing and media, and 802.11 handles wireless networking.

The main idea being tested is how VLANs are carried across trunk links and how that tagging is implemented at Layer 2. The standard that defines VLAN tagging on trunk links and adds a 4-byte tag to Ethernet frames is IEEE 802.1Q. It inserts a VLAN tag into every frame that travels a trunk, allowing a single physical link to carry traffic for multiple VLANs. The tag sits between the source MAC address and the EtherType/length field, so the frame becomes 4 bytes longer. Inside that tag is the Tag Control Information, which includes the VLAN ID (12 bits) and priority bits (for traffic prioritization), plus a bit for drop eligibility. Frames carrying this tag are identified as VLAN-tagged by the Ethertype value 0x8100, after which the original EtherType/length field appears again. This mechanism is what enables switches to keep traffic segregated by VLAN as it traverses trunk links, while still using a single physical connection.

Other options cover different areas: 802.1X is about authenticating devices on a port, 802.3 is the base Ethernet standard for framing and media, and 802.11 handles wireless networking.