CCNA Cisco Certified Network Associate Study Guide - FTP Server

34 Chapter 1 � Internetworking
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
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 networks, 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 logical, not physical.
Before you learn about these layers and their functions, consider a common
hierarchical design as shown in Figure 1.15. 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.
FIGURE 1.15 Hierarchical network design
Core
layer
Distribution
layer
Access
layer
FDDI Ring
Users’ machines Users’ machines Users’ machines
Workgroups