What best describes the practice of stateful multi-layer inspection?

Stateful multi-layer inspection is best described by its ability to analyze network traffic at multiple layers of the OSI model. This practice involves inspecting packets across the different layers to understand the state and context of the traffic, as well as to make more informed security decisions.

In stateful inspection, the firewall keeps track of the state of active connections and uses this information to determine whether a packet is part of an existing connection or a new one. By examining all layers, the inspection can assess not only the headers (like in traditional packet filtering) but also the data within the packet, allowing for better detection of threats and improved management of legitimate traffic.

Other options provided do not accurately reflect the principles of stateful multi-layer inspection. For example, using VLANs relates to network segmentation and access management rather than stateful inspection. Prioritizing specific types of traffic, such as voice and video, pertains to traffic management rather than inspection practices. Similarly, employing Quality of Service (QoS) focuses on bandwidth allocation and does not encompass the complexities involved in stateful multi-layer inspection.