Understanding the Cisco

Understanding the Cisco
Hierarchical Model
Most of us learned about hierarchy early in life. Anyone with older siblings learned what it was
like to be at the bottom of the hierarchy! Regardless of where we were first exposed to hierarchy,
most of us experience it in many aspects of our lives. Hierarchy helps us to understand
where things belong, how things fit together, and what functions go where. It brings order and
understandability to otherwise complex models. If you want a pay raise, hierarchy dictates that
you ask your boss, not your subordinate. That is the person whose role it is to grant (or deny)
your request.
Hierarchy has many of the same benefits in network design that it has in other areas. When
used properly in network design, it makes networks more predictable. It helps us to define and
expect at which levels of the hierarchy we should perform certain functions. You would ask
your boss, not your subordinate, for a raise because of their respective positions in the business
hierarchy. The hierarchy requires that you ask someone at a higher level than yours. Likewise,
you can use tools such as access lists at certain levels in hierarchical networks and you must
avoid them at others.
Let’s face it, large networks can be extremely complicated, with multiple protocols, detailed
configurations, and diverse technologies. Hierarchy helps us to summarize a complex collection
of details into an understandable model. Then, as specific configurations are needed, the model
dictates the appropriate manner for them to be applied.

The Cisco hierarchical model is used to help you design a scalable, reliable, cost-effective
hierarchical internetwork. Cisco defines three layers of hierarchy, as shown in Figure 12.6, each
with specific functionality.