The IEEE 802.1Q standard defines the structure of a VLAN tag, which is inserted into an Ethernet frame. This tag contains a 3-bit field known as the Priority Code Point (PCP). The PCP field is used to mark frames with a specific Class of Service (CoS) priority at Layer 2. With 3 bits available, it is possible to represent 2^3, or 8, distinct priority levels. These levels are numbered from 0 (lowest priority, typically for best-effort traffic) to 7 (highest priority, typically for network control traffic).

B. 16: Incorrect. This would require a 4-bit priority field (2^4), but the IEEE 802.1Q standard specifies only 3 bits for the PCP.

C. 32: Incorrect. This would require a 5-bit priority field (2^5), which is not defined in the 802.1Q VLAN tag structure.

D. 64: Incorrect. This corresponds to a 6-bit field (2^6), which is used for Differentiated Services Code Point (DSCP) in the IP header (Layer 3), not the VLAN header (Layer 2).

1. IEEE Std 802.1Q-2018

IEEE Standard for Local and metropolitan area networks—Bridges and Bridged Networks

IEEE

2018

DOI: 10.1109/IEEESTD.2018.8623023.

Section 9.6

"Tag control information": This section explicitly defines the format of the Tag Control Information (TCI) field within the VLAN tag. It specifies that the Priority Code Point (PCP) is a 3-bit field

stating

"The 3-bit PCP field specifies a priority level for the frame."

2. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson.

Chapter 6

Section 6.4.2

"Virtual Local Area Networks (VLANs)": This textbook

widely used in university networking courses

describes the 802.1Q frame format. It details the 4-byte VLAN tag

including the "3-bit priority field" which allows for the classification of frames into one of eight priority classes.

3. Ericsson. (2019). IP Operating System CLI Command Reference

Quality of Service. 2/221 04-FAP 130 319 Uen Rev C.

Section "QoS Overview"

Subsection "Classification": This official Ericsson documentation states

"Layer 2 classification is based on the 3-bit Priority Code Point (PCP) field in the IEEE 802.1Q VLAN header. The PCP field is also referred to as the 802.1p priority." This confirms the use of a 3-bit field

resulting in 8 priority levels.