The Cisco hierarchical model

The Cisco hierarchical model
The three layers are as follows:
 Core
 Distribution
 Access
Each layer has specific responsibilities. Remember, however, that the three layers are logical
and not necessarily physical. “Three layers” does not necessarily mean “three separate devices.”
Consider the OSI model, another logical hierarchy. The 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 you build physical implementations of hierarchical networks, you might have many
devices in a single layer, or you might have a single device performing functions at two layers.
The definition of the layers is logical, not physical.
Before we examine these layers and their functions, consider a common hierarchical design,
as shown in Figure 12.7. The phrase “keep local traffic local” has almost become a cliché in the
networking world. However, the underlying concept has merit. Hierarchical design lends itself
perfectly to fulfilling this concept. Now, let’s take a closer look at each of the layers.