In a VLAN Tag added to a frame header, the __________ is a 2 byte number that identifies a frame as tagged.

Question

Options

• AUser_priority
• BCanonical Format Indicator
• CVLAN identifier
• DTag Protocol Identifier
• ETag Control information

• A
  4% (1)
• B
  4% (1)
• D
  92% (23)

Explanation

Tag Protocol Identifier (TPID) is the 2-byte field (with a fixed value of 0x8100) inserted at the start of an 802.1Q VLAN tag that signals to receiving devices "this frame has a VLAN tag." Without it, a switch would not know the frame carries tagging information at all.

The distractors are all real fields but serve different roles: Tag Control Information (E) is the other 2-byte component of the tag that contains the remaining fields - it's the container, not the identifier. VLAN Identifier (C) is a 12-bit field inside the TCI that names which VLAN the frame belongs to. User Priority (A) is a 3-bit QoS field also inside the TCI. Canonical Format Indicator (B) is a single bit inside the TCI indicating MAC address byte order.

Memory tip: Think of TPID as the "label on the envelope" - it tells you the envelope is a VLAN-tagged frame before you even open it to read the VLAN number (VID) or priority bits inside.

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