Understanding the OSI Reference Model

The following table outlines the CCNA exam objectives that are reviewed in this chapter. The numbers shown correspond to the master list of objectives found in Chapter 1, “What Is CCNA?”

In years past, the need to understand the Open Systems Interconnection (OSI) reference model for networking grew rapidly. The U.S. government passed laws requiring vendors to support OSI software on their systems or they would no longer buy the systems. Several vendors even predicted that the global Internet would evolve towards using OSI as the protocol instead of TCP/IP. As we near the turn of the century, however, OSI has been implemented on a much smaller scale than predicted. Few vendors push their OSI software solutions, if they have them. Several components of the OSI model are popularly implemented today. For example, OSI NSAP network layer addresses are often used for signaling in Asynchronous Transfer Mode (ATM) networks. However, full seven-layer OSI implementations are relatively rare today.

So why have a whole chapter on OSI? The biggest reason is that the OSI seven-layer reference model is an excellent point of reference for describing the concepts and functions behind other network protocol implementations. References to Layer 2 switching and Layer 3 switching, which are popular topics today, refer to the comparison to Layers 2 and 3 of the OSI model. Cisco courses make generous use of the OSI model as reference for comparison of other network protocol implementations. So, this chapter will not actually help you understand OSI fully, but rather it will discuss OSI functions in comparison with popularly implemented protocols.