2.4 2-tier
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. In this architecture a presentation layer or interface runs on a client, and a data layer or data structure gets stored on a server. Separating these two components into different locations represents a two-tier architecture. It's also known as collapsed core design because it's only 2 layers. In this the distribution layer is merged with the core layer.
