3 Layer 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.

Each layer in the three-tier hierarchical model has a unique role to perform: 

•Access Layer—The primary function of an access-layer is to provide network access to the end user. This layer often performs OSI Layer-2 bridge function that interconnects logical Layer-2 broadcast domains and provides isolation to groups of users, applications, and other endpoints. The access-layer interconnects to the distribution layer.

•Distribution Layer—Multi-purpose system that interfaces between access layer and core layer. Some of the key function for a distribution layer include the following:

–Aggregate and terminate Layer-2 broadcast domains

–Provide intelligent switching, routing, and network access policy function to access the rest of the network.

–Redundant distribution layer switches provides high availability to the end-user and equal-cost paths to the core. It can provide differentiated services to various class-of-service applications at the edge of network.

•Core Layer—The core-layer provides high-speed, scalable, reliable and low-latency connectivity. The core layer aggregates several distribution switches that may be in different buildings. Backbone core routers are a central hub-point that provides transit function to access the internal and external network.

 http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Education/SchoolsSRA_DG/SchoolsSRA-DG/SchoolsSRA_chap3.html