4444.4444THE BRIDGE DECIDES TO FILTER (NOT FORWARD) THE FRAME THA...

0200.4444.4444

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 Hub1. The bridge 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 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 E1 interface. So, the

bridge forwards the frame.

Transparent Bridging 239

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.

How Bridges Learn MAC Addresses

Bridges perform three main functions, as mentioned earlier. One of those key functions is to

learn the MAC addresses in the network to build its bridging table. With a full, accurate

bridging table, the bridge can make accurate forwarding and filtering decisions.

Bridges build the bridge table by listening to incoming frames and examining the source

MAC address in the frame. If a frame enters the bridge and the source MAC address is not

in the bridge table, the bridge creates an entry in the table. The MAC address is placed into the

table, along with the interface in which the frame arrived. Bridge learning logic is that simple.

Figure 9-4 depicts the same network as Figure 9-3, but before the bridge has built any bridge

table entries. In the figure, the first two frames sent in this network are shown—first a frame

from Fred, addressed to Barney, followed by Barney’s response, addressed to Fred.

Figure 9-4

Bridge Learning: Empty Table and Adding Two Entries

Bridge Table: Before Either Frame is sent

I learned Fred’s MAC when he sent