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