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.
See also 
- Khalid Raza, Mark Turner (2002), Cisco Network Topology and Design, Cisco Press
- High Availability Campus Network Design, retrieved 2010-08-23
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