The ___________ is a single-bit flag that indicates that the format of MAC address information is presented in a standard form.

Question

Options

• ATag Control Information
• BUser_Priority
• CVLAN Identifier
• DTag Protocol Identifier
• ECanonical Format Indicator

• B
  2% (1)
• C
  4% (2)
• D
  2% (1)
• E
  92% (46)

Explanation

Canonical Format Indicator (CFI) is the only single-bit field in the IEEE 802.1Q VLAN tag structure specifically designed to signal whether MAC addresses are presented in canonical (standard Ethernet) byte order - a 0 means canonical, a 1 means non-canonical (used historically for Token Ring interoperability).

The distractors are wrong for these reasons:

• A. Tag Control Information (TCI) is the full 3-byte container that holds the CFI along with other fields - it's not a flag itself.
• B. User_Priority is a 3-bit field (not 1-bit) used for QoS/CoS traffic prioritization (values 0–7).
• C. VLAN Identifier (VID) is a 12-bit field that identifies which VLAN a frame belongs to (supports up to 4094 VLANs).
• D. Tag Protocol Identifier (TPID) is a 16-bit field set to 0x8100 that marks the frame as 802.1Q-tagged.

Memory tip: CFI = Check Format Immediately - it's the one-bit "checker" that tells you if the MAC address format is standard. If you remember it's the only single-bit field in the tag, you can eliminate all other options by their bit-widths (3, 12, 16 bits) alone.

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