hannel. A single physical interface can be a backup to an EtherChannel as well. The backup link does not need to be the same type of interface as the primary. For example, a 100 Mbps interface can be a backup for a 1 Gbps interface. Monitor flex links with the show interface switchport backup command: 3750#sho int switchport backup Switch Backup Interface Pairs: Active Interface Backup Interface State GigabitEthernet1/0/20 GigabitEthernet1/0/21 Active Down/Backup Down Chapter 19: Catalyst 3750 Features Storm Control Storm control prevents broadcast, multicast, and unicast storms from overwhelming a network. Storms can be the result of a number of issues, from bridging loops to virus outbreaks. With storm control, you can limit the amount of storm traffic that can come into a switch port. Outbound traffic is not limited. With storm control enabled, the switch monitors the packets coming into the config- ured interface. It determines the amount of unicast, multicast, or broadcast traffic every 200 milliseconds, then compares that amount with a configured threshold. Packets that exceed the threshold are dropped. This sounds straightforward, but the feature actually works differently from how many people expect. When I first learned of it, I assumed that the preceding description accurate-that is, that at any given time, traffic of the type I'd configured for monitoring would be allowed to come into the switch until the threshold was met (similar to what is shown in Figure 19-1). The reality, however, is more complicated. Bandwidth Threshold Figure 19-1. Incorrect storm-control model In reality, the switch monitors the interface, accumulating statistics in 200 ms incre- ments. If, at the end of 200 ms, the threshold has been exceeded, the configured (or default) action is taken for the next 200 ms increment. Figure 19-2 shows how storm-control actually functions. Traffic is measured in 200 ms increments, shown on the graph as TO, T1, and SO on. If the type of traffic being moni- tored does not surpass the configured 