The Forward Versus Filter Decision

Transparent bridges reduce collisions by forwarding traffic from one segment to the other only when necessary. To decide whether to forward a frame, the bridge uses a dynamically built table, called a bridge table. The bridge examines the bridging table to decide whether it should forward a frame. For example, consider the simple network shown in Figure 9-3, with Fred first sending a frame to Barney and then one to Wilma.

Figure 9-3 Example Transparent Bridging Forwarding and Filtering Decision

Fred 0200.1111.1111

Barney 0200.2222.2222

Fred 0200.1111.1111

Barney 0200.2222.2222

Wilma 0200.3333.3333

Figure 9-3 Example Transparent Bridging Forwarding and Filtering Decision

Fred 0200.1111.1111

Wilma 0200.3333.3333

Barney 0200.2222.2222

Betty 0200.4444.4444

Betty 0200.4444.4444

Wilma 0200.3333.3333

Fred 0200.1111.1111

Wilma 0200.3333.3333

Barney 0200.2222.2222

 Bridge Table 0200.1111.1111 E0 0200.2222.2222 E0 0200.3333.3333 E1 0200.4444.4444 E1

The bridge decides to filter (not forward) the frame that Fred sends to Barney. Fred sends a frame with the destination MAC address of 0200.2222.2222, which is Barney's MAC address. The bridge overhears the frame because it is attached to Hubl. The bridge then decides what common sense tells you from looking at the figure—it should not forward the frame because Barney, attached to Hubl as well, already will have received the frame. But how does the bridge know to make that decision? The bridge decides to filter—in other words, not forward—the frame because it received the frame on port E0, and it knows that Barney's MAC also is located out E0.

Conversely, the bridge decides to forward the frame that Fred sends to Wilma in the lower part of the figure. The frame enters the bridge's E0 interface, and the bridge knows that the destination address, 0200.3333.3333, is located somewhere out its El interface. So, the bridge forwards the frame.

The rules for hub behavior have not changed—whenever a frame is sent toward a hub, the hub repeats the frame out every other port.