When an Ethernet switch receives a frame with a priority tag, which standard is it using?

An Ethernet switch that receives a frame with a priority tag is utilizing the IEEE 802.1Q standard. This standard is specifically designed for introducing Virtual LANs (VLANs) and includes the capability for tagging frames with a priority level in what's known as Quality of Service (QoS).

When a frame is tagged, it includes a 4-bit priority code point (PCP) that allows for prioritizing certain types of traffic, which is crucial in environments where bandwidth management and ensuring timely delivery of data packets are essential. This capability enables network devices to differentiate between traffic types, such as voice, video, and standard data, ensuring that more sensitive data can be prioritized over less critical transmissions.

In contrast, other options focus on different aspects of networking. IEEE 802.1D addresses bridging and spanning tree protocols, IEEE 802.3ad relates to link aggregation for increased bandwidth and redundancy, and IEEE 802.3 covers standards for Ethernet networking without specific provisions for VLAN tagging or QoS. Therefore, the presence of a priority tag aligns specifically with the functions defined in IEEE 802.1Q.