The Cisco Hierarchical Design Model is another network model that is used to design and engineer data communication networks. The Hierarchical Design Model is a three-tiered, or layered, model with a core, distribution, and access layer, as illustrated in Figure 2-3.
The Hierarchical Design Model is used by network designers, architects, and engineers when designing and implementing scalable and efficient networks. As stated previously, the three tiers of this model (illustrated in Figure 2-4) are the core, distribution, and access layers. The core layer provides high-speed switching between sites and is considered the backbone of the network. The distribution layer provides policy-based connectivity, such as what type of data can and cannot transit across the network. The access layer enables users to access the network and its resources.
Hierarchical design models can also be found in travel. The taxi you take from home to the airport is working at the access layer because the taxi is providing access to the airport resources (in this case, the airplane). At the airport, your ticket determines through which gate you enter. Your ticket provides the routing?that is, it tells you which gate to use to board your airplane.