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.