Company XYZ is designing their network using the three-layer hierarchical model. At which layer must the QoS design classify or mark the traffic?

The access layer is where end devices connect to the network and is the earliest point where traffic can be identified by application type, user, or device. Classifying and marking at ingress here establishes a trust boundary, allowing all upstream layers to simply honor the DSCP or CoS markings without repeating expensive deep packet inspection, and prevents untrusted end devices from self-marking traffic with elevated priority.

The distribution layer enforces and applies QoS policies such as queuing and policing but relies on markings already applied at the access layer rather than performing initial classification.

The core layer is designed for high-speed packet forwarding with minimal processing; performing classification here violates the core design principle and introduces latency at the highest-bandwidth segment of the network.

A collapsed core combines core and distribution functions but QoS classification still must originate at the access layer where traffic enters the network, not at the aggregation point.