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http://www.cisco.com/univercd/cc/td/doc ... ansbdg.htm
Transparent Bridging Operation
Transparent bridges are so named because their presence and operation are transparent to network hosts. When transparent bridges are powered on, they learn the workstation locations by analyzing the source address of incoming frames from all attached networks. For example, if a bridge sees a frame arrive on port 1 from Host A, the bridge concludes that Host A can be reached through the segment connected to port 1. Through this process, transparent bridges build a table (the learning process), such as the one in Figure 23-1.