Which of the following are characteristics of 802.1p? (Choose two)

Question

Options

• A802.1p adds 16 bits to the Layer 2 header,
• B802.1p adds 16 bits to the Layer 3 header.
• C802.1p specifies 64 different priority levels.
• D802.1p uses a field in the 802.1Q header.
• E802.1p uses a field in the Layer 3 IP header
• F802.1p defines a 3-bit Class of Service field.

• B
  6% (1)
• D
  94% (17)

Explanation

D and F are correct because 802.1p does not introduce its own header - instead it repurposes the existing 3-bit Priority Code Point (PCP) field inside the 802.1Q VLAN tag to mark Layer 2 traffic with one of 8 priority levels (0–7), which is what the "Class of Service" (CoS) field refers to.

Why the distractors are wrong:

• A & B: 802.1p adds no bits on its own - it borrows from the 802.1Q tag (which adds 32 bits, not 16). B is doubly wrong because 802.1p is Layer 2, not Layer 3.
• C: A 3-bit field yields only 2³ = 8 levels, not 64. (64 would require 6 bits.)
• E: Layer 3 QoS uses DSCP in the IP header - that is not 802.1p.

Memory tip: Think of 802.1p as "Q's 3-bit passenger" - 802.1Q owns the VLAN tag, and 802.1p hitches a ride using just 3 bits of it for priority. If you see Layer 3 or a number other than 8 (or 3), it's a trap.

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