More network basic content can be found at:
4.2 Inter-VLAN Routing
4.2.1 Network Layering
The three-tier network structure is a three-tier network using a hierarchical architecture. The three-tier network architecture adopts a hierarchical model design, that is, the complex network design is divided into several levels, and each level focuses on certain specific functions, so that a complex big problem can be turned into many simple small problems.
The network designed by the three-layer network architecture has three layers: the core layer (the high-speed switching backbone of the network), the aggregation layer (providing policy-based connections), and the access layer