CHAPTER Bridges and layer 2 switches 6 INTRODUCTION Bridges and layer 2 switches are network systems that interconnect networks at the Data Link layer of the OSI Reference Model. Considering the classification of net- work systems presented in Chapter 2, bridges and layer 2 switches implement a two-layer protocol stack for each network attachment, as shown in Figure 6-1, and switch frames among these stacks. Layer 2 switches can be considered a special implementation of bridges and thus can be viewed as a subset of bridging systems. The following discussion uses the term bridge to refer to all data link layer internetworking systems and describes the specifics of layer 2 switches at the end of the chapter. Figure 6-1 shows the route of a frame traversing two networks attached to the