This chapter provides an overview of transparent bridging's handling of traffic and protocol components. Transparent bridges are so named because their presence and operation are transparent to network hosts. When transparent bridges are powered on, they learn the network's topology by analyzing the source address of incoming frames from all attached networks. If, for example, a bridge sees a frame arrive on Line 1 from Host A, the bridge concludes that Host A can be reached through the network connected to Line 1. Through this process, transparent bridges build a table, such as the one in Figure 26-1 .
Transparent bridges build a table that determines a host's accessibility.
The bridge uses its table as the basis for traffic forwarding. When a frame is received on one of the bridge's interfaces, the bridge looks up the frame's destination address in its internal table. If the table contains an association between the destination address and any of the bridge's ports aside from the one on which the frame was received, the frame is forwarded out the indicated port. If no association is found, the frame is flooded to all ports except the inbound port. Broadcasts and multicasts also are flooded in this way.