Cisco NM-8B-U User Guide - Page 39

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series Feature Overview Global storm-control monitors incoming traffic statistics over a time period and compares the measurement with a predefined suppression level threshold. The threshold represents the percentage of the total available bandwidth of the port. If the threshold of a traffic type is reached, further traffic of that type is suppressed until the incoming traffic falls below the threshold level. Global storm-control is disabled by default. The switch supports global storm-control for broadcast, multicast, and unicast traffic. This example of broadcast suppression can also be applied to multicast and unicast traffic. The graph in Figure 18 shows broadcast traffic patterns on an interface over a given period of time. In this example, the broadcast traffic exceeded the configured threshold between time intervals T1 and T2 and between T4 and T5. When the amount of specified traffic exceeds the threshold, all traffic of that kind is dropped. Therefore, broadcast traffic is blocked during those intervals. At the next time interval, if broadcast traffic does not exceed the threshold, it is again forwarded. Figure 18 Broadcast Suppression Example Total number of broadcast packets or bytes Threshold Forwarded traffic Blocked traffic 46651 0 T1 T2 T3 T4 T5 Time When global storm-control is enabled, the switch monitors packets passing from an interface to the switching bus and determines if the packet is unicast, multicast, or broadcast. The switch monitors the number of broadcast, multicast, or unicast packets received within the 1-second time interval, and when a threshold for one type of traffic is reached, that type of traffic is dropped. This threshold is specified as a percentage of total available bandwidth that can be used by broadcast (multicast or unicast) traffic. The combination of broadcast suppression threshold numbers and the 1-second time interval control the way the suppression algorithm works. A higher threshold allows more packets to pass through. A threshold value of 100 percent means that no limit is placed on the traffic. Note Because packets do not arrive at uniform intervals, the 1-second time interval during which traffic activity is measured can affect the behavior of global storm-control. The switch continues to monitor traffic on the port, and when the utilization level is below the threshold level, the type of traffic that was dropped is forwarded again. You use the storm-control broadcast, storm-control multicast, and storm-control unicast interface configuration commands to set up the global storm-control threshold value. Global storm-control and per-port storm-control cannot be enabled at the same time. Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ 39