The primary function of the distribution layer is to provide routing, filtering, WAN access and to ______.

• A. provide secure user authentication for devices
• B. manage access to external networks
• C. control end-user access to applications
• D. determine how packets can access the core when necessary

Answer: (D)

In a three-layer network, the distribution layer sits between the access layer and the core and acts as the policy boundary. It handles routing between VLANs, applies filtering (like ACLs), and provides WAN access, but crucially it controls what traffic is allowed to reach the core. Saying it determines how packets can access the core captures that gatekeeping and policy-enforcement role: traffic is examined and routed toward the backbone only when permitted. The other options describe tasks better handled by authentication services at the edge, general user-to-application access controls, or border/edge functions, rather than the distribution layer’s responsibility to regulate core entry.