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Chapter 1

The Campus Network

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 logi-
cal 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 hierar-
chical networks, you may 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 1.7. 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 ful-
filling this concept. Now, let's take a closer look at each of the layers.

F I G U R E 1 . 7

A hierarchical network design

Core
layer

Distribution
layer

Access
layer

FDDI Ring

Users' machines

Workgroups

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