LAN Switching

Before bridges were created, a 10BASE-T network might have begun to suffer from performance problems. As described in the previous section, to improve performance, you might have added a two-port bridge, created two LAN segments, doubled the bandwidth, reduced collisions, and improved performance.

Now take a step back and think about what might happen to that network with the bridge 6 months later. More devices have been added to the segments on each side of the bridge. More bandwidth-hungry applications have been added. Eventually, both LAN segments might become as congested as the original single Ethernet segment was 6 months earlier.

What's the solution? What about a four-port bridge? The engineer adds the four-port bridge, converting the two segments to four segments, again doubling bandwidth, and again reducing collisions. A few months later, the number of devices has increased, more bandwidth-hungry applications have been added, and you need an eight-port bridge! You can see a vicious cycle beginning to occur.

From one perspective, switches are bridges with lots of ports. Switches behave identically to transparent bridges in terms of forwarding and learning, but switches typically have many more ports and much faster internal processing. So, if a campus network needed to be broken into 100 different segments, you could use a switch with 100 ports in it. It would break the

Ethernet into 100 different collision domains, or segments, and create 100 different sets of 10-Mbps bandwidth (or more, if Fast Ethernet or Gigabit Ethernet were used). It again would reduce collisions, just like bridges. In short, switches do the same thing as bridges, only faster and better. In fact, an old saying says it best: "Switches are bridges on steroids."

So, if bridges and switches do the same things the same way, why have two names? There were many reasons, none of which matters for the CCNA exams. Today you do not even have to choose between buying a bridge or a switch—vendors sell only switches.

The following list provides a quick review of the basic forwarding logic used by a switch or bridge:

1. A frame is received.

2. If the destination is a broadcast or multicast, forward on all ports except the port in which the frame was received.

3. If the destination is a unicast and the address is not in the address table, forward on all ports except the port in which the frame was received.

4. If the destination is a unicast and the address is in the address table, and if the associated interface is not the interface in which the frame arrived, forward the frame out the one correct port.

5. Otherwise, filter (do not forward) the frame.

For instance, in Figure 9-5, the network has been migrated to use a switch. The switch's bridging table already has been populated with all the MAC addresses in the network. Fred sends another frame to Barney. The switch knows that Barney is located off his E1 port, so the switch forwards the frame out E1.

Figure 9-5 Example: Forwarding Logic for a Switch

Figure 9-5 Example: Forwarding Logic for a Switch

Wilma 0200.3333.3333

0200.1111.1111

E1 E3

0200.1111.1111

Wilma 0200.3333.3333

Barney 0200.2222.2222

Betty 0200.4444.4444

E1 E3

Bridge Table

0200.1111.1111

E0

0200.2222.2222

E1

0200.3333.3333

E2

0200.4444.4444

E3

Although the basic operation of bridges and switches is identical, switches do differ from transparent bridges in some regards. Some of the differences exist just because newer features were introduced to the market around the same time that switches became popular. Other features, such as the optimized internal processing on switches, do create a significant advantage to switches over bridges. Practically, the differences do not really matter because vendors continue to improve and develop features for switches, whereas it is hard to find vendors that offer products called bridges anymore.

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