1.3 LAN Switching and the Hierarchical Model of Network Design
1.3.1 Overview
A hierarchical network design includes the following three layers: 
  • The core layer, which provides optimal connectivity between distribution blocks
  • The distribution layer, which provides policy-based connectivity 
  • The access layer, which provides access-layer aggregation and L3/L4 services

The Figure shows a high-level view of the various aspects of a hierarchical network design. A hierarchical network design presents three layers - core, distribution, and access - with each layer providing different functionality.