The
three-tier network model: Core (high-speed backbone, minimal filtering), Distribution (routing, ACLs, inter-VLAN routing, policy enforcement), Access (end devices connect, port security, VLANs, PoE).
Core = speed (no complex processing). Distribution = policy (ACLs, routing between VLANs, QoS). Access = connectivity (user/device facing, port security, 802.1X). Security controls belong at the distribution layer primarily. Collapsed core/distribution is common in smaller environments.