In the Cisco three-layer hierarchical design model (Access, Distribution, Core), the access layer is primarily responsible for:

Providing user/device connectivity.

Enforcing policy boundaries.

Performing QoS classification and marking at ingress.

The access layer is the optimal point to classify and mark traffic for QoS because:

It is the first point of entry into the network for end-user devices.

Markings can then be trusted by upstream devices (distribution and core).

This minimizes trust boundaries throughout the network and ensures consistent policy enforcement.

This design principle ensures end-to-end QoS consistency, a key objective emphasized in CCDE v3.1 for scalable and maintainable enterprise designs.

Why other options are incorrect:

A (Fault isolation): Typically handled at distribution/core layers.

C (Reliability): A general network-wide requirement, not specific to access.

D (Fast transport): Primarily a core layer function.

E (Redundancy and load balancing): Managed primarily at distribution and core layers.