The three-tier hierarchical design maximizes performance, network availability, and the ability to scale the network design.
However, many small enterprise networks do not grow significantly larger over time. Therefore, a two-tier hierarchical design where the core and distribution layers are collapsed into one layer is often more practical. A “collapsed core” is when the distribution layer and core layer functions are implemented by a single device. The primary motivation for the collapsed core design is reducing network cost, while maintaining most of the benefits of the three-tier hierarchical model.
The example in the figure has collapsed the distribution layer and core layer functionality into multilayer switch devices.
The hierarchical network model provides a modular framework that allows flexibility in network design and facilitates ease of implementation and troubleshooting.