QoS defines policies on how switches and routers deliver different types of traffic. A QoS domain is the entire collection of network devices that are administered so that they adhere to the QoS policies.
To guarantee that QoS policies are met, QoS must be configured on all switches and routers end-to-end across the network.
Traffic should be classified at the edges of the QoS domain. Where this isn't possible, classify traffic as close as possible to the source. Classification can occur at Layer 2 or Layer 3, depending on the network functions available at the edge.
The top portion of Figure 13-1 shows QoS operations on a Catalyst switch, including the following: