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The Cisco Three-Layer Model
F I G U R E 1 . 3
The Cisco hierarchical model
The three layers are:
The Core layer
The Distribution layer
The Access layer
Each layer has specific responsibilities. Remember, however, that the
three layers are logical and not necessarily physical. Three layers do not nec-
essarily mean three separate devices. Consider the OSI model, another logical
hierarchy. Its seven layers describe functions but not necessarily protocols,
right? Sometimes a protocol maps to more than one layer of the OSI model,
and sometimes multiple protocols communicate within a single layer. In the
same way, when we build physical implementations of hierarchical net-
works, we may have many devices in a single layer, or we might have a single
device performing functions at two layers. The definition of the layers is log-
ical, not physical.
Before you learn about these layers and their functions, consider a com-
mon hierarchical design, as shown in Figure 1.4. The phrase "keep local traf-
fic local" has almost become a clichй in the networking world. However, the
underlying concept has merit. Hierarchical design lends itself perfectly to ful-
filling this concept.
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