The Evolving Network Model - The Hierarchical Design Model

Cisco used the three-level Hierarchical Design Model for years. This older model provided a high-level idea of how a reliable network might be conceived, but it was largely conceptual because it didn’t provide specific guidance. Figure 1-1 shows the Hierarchical Design Model.

Figure 1-1 Hierarchical Design Model












Figure 1-2 is a simple drawing of how the three-layer model might have been built out. A distribution layer-3 switch is used for each building on campus, tying together the access switches on the floors. The core switches link the various buildings together.

This same three-layer hierarchy can be used in the WAN with a central headquarters, division headquarters, and units.

Figure 1-2 Three-Layer Network Design













The layers break a network in the following way:
■ Access layer—End stations attach to the network using low-cost devices.
■ Distribution layer—Intermediate devices apply policies.
— Route summarization
— Policies applied, such as:
• Route selection
• Access lists
• Quality of Service (QoS)
■ Core layer—The backbone that provides a high-speed path between distribution elements.
— Distribution devices are interconnected.
— High speed (there is a lot of traffic).
— No policies (it is tough enough to keep up).

Later versions of this model include redundant distribution, core devices, and connections, which make the model more fault-tolerant.

Problems with the Hierarchical Design Model This early model was a good starting point, but it failed to address key issues, such as:
■ Where do wireless devices fit in?
■ How should Internet access and security be provisioned?
■ How do you account for remote access, such as dial-up or VPN?
■ Where should workgroup and enterprise services be located?