Hierarchical internetworking model

The Hierarchical internetworking model, or three-layer model, is a network design model first proposed by Cisco. The three-layer model divides enterprise networks into three layers: core, distribution, and access layer. Each layer provides different services to end-stations and servers.

Access layer  

End-stations and servers connect to the enterprise at the access layer. Access layer devices are usually commodity switching platforms, and may or may not provide layer 3 switching services. The traditional focus at the access layer is minimizing "cost-per-port": the amount of investment the enterprise must make for each provisioned Ethernet port.

 

Distribution layer

The distribution layer is the "smart" layer in the three-layer model. Routing, filtering, and QoS policies are managed at the distribution layer. Distribution layer devices also often manage individual branch-office WAN connections.

Core layer

The core network provides high-speed, highly-redundant forwarding services to move packets between distribution-layer devices in different regions of the network. Core switches and routers are usually the most powerful, in terms of raw forwarding power, in the enterprise; core network devices manage the highest-speed connections, such as 10 Gigabit Ethernet.