The Cisco Three-Layer Hierarchical Model
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It helps us define which areas should perform certain functions. Likewise,
you can use tools such as access lists at certain levels in hierarchical networks
and 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 summarize a complex collection of details into an understandable model.
Then, as specific configurations are needed, the model dictates the appropri-
ate manner to apply them.
The Cisco hierarchical model can help you design, implement, and main-
tain a scalable, reliable, cost-effective hierarchical internetwork. Cisco
defines three layers of hierarchy, as shown in Figure 1.23, each with specific
functions.
F I G U R E 1 . 2 3
The Cisco hierarchical model
The following are the three layers:
The core layer
The distribution layer
The access layer
Each layer has specific responsibilities. Remember, however, that the
three layers are logical and are not necessarily physical devices. Consider the
OSI model, another logical hierarchy. The seven layers describe functions
Core
layer
Distribution
layer
Access
layer
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