Chapter 12. Protecting against bridging loops by using the Spanning Tree Protocol
In the last chapter, you connected two switches—Switch1 and Switch2—via a single Ethernet connection. Imagine for a moment that both of these switches are in a live network and have dozens of end users connected to them. If the link between the switches goes down, devices on Switch1 won’t be able to communicate with devices on Switch2 and vice versa.
The solution is to add a redundant connection between FastEthernet0/23 on Switch1 and Switch2, as shown in figure 12.1. If the original link fails for whatever reason, the switches can communicate across the redundant link.
Figure 12.1. Switch1 and Switch2 with redundant links. Note that the interface numbers are the same on each end of the link.
At first glance, this looks like a wonderful configuration. It appears that by adding an additional connection, you’re adding not only redundancy but additional bandwidth as well. But this configuration isn’t as wonderful as it looks.
When you add a redundant connection, by default you can’t have traffic traversing both links simultaneously. To understand why, consider the following scenario.
Suppose that Executive-PC1 generates a broadcast Ethernet frame and sends it out onto the network. Recall from chapter 2 that Switch2 will forward such a broadcast out every port, including both ports connected to Switch1—FastEthernet0/23 and FastEthernet0/24. Figure 12.2 illustrates what this would look like.