Bridges

Bridges are thought of as Data Link Layer devices because they separate collision domains. According to 802.1D, the standards document that describes the operation of a bridge (relay agent), bridges actually operate at the MAC Layer as shown in Figure 5-2. Bridge management and other aspects of a bridge may operate at higher layers, such as the LLC Layer, but the relay agent operates at the MAC Layer. Because the job of an Ethernet bridge is to forward traffic between collision domains, they are discussed here instead of in the previous chapter that described activities within a collision domain. A switch may be described as a multiport bridge. The term bridge is used instead of switch because switches are not always OSI Layer 2 devices but bridges are.

Figure 5-2. Bridge (relay agent) placement in the OSI Model