Cisco NM-8B-U - HW ROUTERS L-M User Manual page 39

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
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
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
Total
number of
broadcast
packets
or bytes
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.
Because packets do not arrive at uniform intervals, the 1-second time interval during which traffic
Note
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.
Figure 18 shows broadcast traffic patterns on an interface over a given period of time. In this
Broadcast Suppression Example
0 T1 T2 T3
Threshold
T4 T5 Time
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
Forwarded traffic
Blocked traffic
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