Which field in the 802.1Q tag is used for Class of Service?

The field in the 802.1Q tag that is used for Class of Service is the PCP, or Priority Code Point. This 3-bit field allows for the classification of traffic into different priority levels, enabling the implementation of Quality of Service (QoS) in a network. The different priorities facilitate the management of bandwidth and latency, essentially determining how packets should be handled in times of congestion or high traffic loads.

The PCP field supports up to eight different priority levels, which network devices can use to prioritize and manage traffic effectively. This is essential in environments that require specific performance standards for different types of data, such as voice, video, or critical application data.

The other fields serve different purposes: for instance, the VID (VLAN Identifier) is used to identify the VLAN to which the frame belongs, and the DEI (Drop Eligible Indicator) is utilized to indicate if a frame is eligible to be dropped under certain network conditions. The TPID (Tag Protocol Identifier) indicates the EtherType value being used to signify that the following fields are part of an 802.1Q header. Each of these fields plays a vital role in VLAN tagging and traffic management but does not specifically address the Class of Service as the PCP does.