Switches reduce network overhead by forwarding traffic from one segment to another only when necessary. To decide whether to forward a frame, the switch uses a dynamically built table called a bridge table or MAC address table. The switch examines the address table to decide whether it should forward a frame. For example, consider the simple network Fred first sends a frame to Barney and then one to Wilma. The switch decides to filter (in other words, to not forward) the frame that Fred sends to Barney. Fred sends a frame with a destination MAC address of 0200.2222.2222, which is Barney’s MAC address. The switch overhears the frame, because it is attached to Hub1.
The switch then decides what common sense tells you from looking at the figure—it should not forward the frame, because Barney, attached to Hub1 as well, already has received the frame. (Hubs simply repeat the signal out all ports, for all frames, so the switch receives everything sent by either Barney or Fred.) But how does the switch know to not forward the frame? The switch decides to filter the frame because it received the frame on port E0, and it knows that Barney’s MAC is also located out E0. Conversely, the switch decides to forward the frame that Fred sends to Wilma in the bottom part of the figure. The frame enters the switch’s E0 interface, and the switch knows that the destination address, 0200.3333.3333, is located somewhere out its E1 interface. So the switch forwards the frame.
The switch then decides what common sense tells you from looking at the figure—it should not forward the frame, because Barney, attached to Hub1 as well, already has received the frame. (Hubs simply repeat the signal out all ports, for all frames, so the switch receives everything sent by either Barney or Fred.) But how does the switch know to not forward the frame? The switch decides to filter the frame because it received the frame on port E0, and it knows that Barney’s MAC is also located out E0. Conversely, the switch decides to forward the frame that Fred sends to Wilma in the bottom part of the figure. The frame enters the switch’s E0 interface, and the switch knows that the destination address, 0200.3333.3333, is located somewhere out its E1 interface. So the switch forwards the frame.