Filtering

What happens when Station 2 in Figure 3-2 responds to Station 1? All stations on the segment off port A.1, including Bridge A, receive the frame. Bridge A learns about the presence of Station 2 and adds its MAC address to the bridge table along with the port identifier (A.1). Bridge A also looks at the destination MAC address to determine where to send the frame. Bridge A knows Station 1 and Station 2 exist on the same port. It concludes that it does not need to send the frame anywhere. Therefore, Bridge A filters the frame. Filtering occurs when the source and destination reside on the same interface. Bridge A could send the frame out other interfaces, but because this wastes bandwidth on the other segments, the bridging algorithm specifies to discard the frame. Note that only Bridge A knows about the existence of Station 2 because no frame from this station ever crossed the bridge.

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