Which VLAN tagging standard header does IEEE 802.1p modify to identify Ethernet frame priority?

The correct answer is associated with the 802.1Q standard, which defines VLAN tagging in Ethernet frames. The modification made by IEEE 802.1p specifically adds a priority code point (PCP) field within the VLAN tag header defined by 802.1Q. This field allows for the prioritization of Ethernet frames based on traffic type, effectively enabling Quality of Service (QoS) in networks.

When a frame is tagged with a VLAN header, the PCP field uses three bits to specify the priority of the frame, facilitating better management of network resources by allowing higher-priority traffic to be transmitted with precedence over lower-priority traffic. This feature is crucial for applications that require reliable performance, such as voice over IP or real-time video streaming.

In contrast, the other standards listed have different focuses. 802.1D pertains to bridging and network management, defining the operation of network bridges. 802.1X is a port-based network access control standard used for authenticating devices on a network. 802.1A does not relate to VLAN tagging and is less commonly referenced in the context of networking standards. Overall, 802.1Q is the foundation for VLAN tagging, and the addition of the 802.