Which of the following statements about 802.1Q is incorrect?

Question

Options

• AThe value of TPID in the 802.1Q Ethernet frame structure is 8100
• BTheoretically, the value range of VLAN ID is 0~4095
• CThere are only 4 VLAN priorities in the 802.1Q Ethernet frame structure
• D802.1Q Ethernet frame is 4 bytes more than ordinary Ethernet frame

• B
  10% (2)
• C
  85% (17)
• D
  5% (1)

Explanation

C is the incorrect statement because the 802.1Q tag contains a 3-bit Priority Code Point (PCP) field, which supports 8 priority levels (0–7), not 4. Confusing 3 bits with 2 bits is a common mistake - 2 bits would give 4 levels, but PCP is 3 bits.

Why the other options are correct:

• A is correct - TPID (Tag Protocol Identifier) is always 0x8100, which identifies the frame as 802.1Q-tagged.
• B is correct - the VID (VLAN ID) field is 12 bits wide, giving 2¹² = 4096 possible values (0–4095), though VLANs 0 and 4095 are reserved.
• D is correct - the 802.1Q tag inserts a 4-byte header (2 bytes TPID + 2 bytes TCI) between the source MAC address and the EtherType field.

Memory tip: Think "3 bits = 8 priorities" by remembering the tag breakdown as 3-1-12 (PCP-DEI-VID). The 3-bit PCP doubles as a reminder that there are 8 (2³) priority levels - the same 8 used in Quality of Service traffic classes.

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