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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter25 Configuring Port-Based Traffic Control
Configuring Storm Control
Storm control uses one of these methods to measure traffic activity:
•Bandwidth as a percentage of the total available bandwidth of the port that can be used by the
broadcast, multicast, or unicast traffic
•Traffic rate in packets per second at which broadcast, multicast, or unicast packets are received
•Traffic rate in bits per second at which broadcast, multicast, or unicast packets are received
With each method, the port blocks traffic when the rising threshold is reached. The port re mains blocked
until the traffic rate drops below the falling threshold (if one is specified) and then resumes normal
forwarding. If the falling suppression level is not specified, the switch blocks all traffic until the traffic
rate drops below the rising suppression level. In general, the higher the l evel, the le ss effective the
protection against broadcast storms.
Note When the storm control threshold for multicast traffic is reached, all multicast traffic except control
traffic, such as bridge protocol data unit (BDPU) and Cisco Discovery Protocol (CDP) frames, are
blocked.However, the switch does not differentiate between routing updates, such as OSPF, and regular
multicast data traffic, so both types of traffic are blocked.
The graph in Figure 25-1 shows broadcast traffic patterns on an interface over a given period of time.
The example can also be applied to multicast and unicast traffic. In this example, the broadcast traffic
being forwarded exceeded the configured threshold between time intervals T1 and T2 and between T4
and T5. When the amount of specified traff ic e x ceeds th e thr esho ld, a ll tra f f i c of that kind i s d roppe d fo r
the next time period. Therefore, broadcast traffic is blocked during the intervals following T2 and T5.
At the next time interval (for example, T3), if broadcast traffic does n ot e x ce ed the th resho ld , it i s ag ain
forwarded.
Figure25-1 Broadcast Storm Control Example
The combination of the storm-control suppression level and the 1-second time interval control s the way
the storm control algorithm works. A higher threshold allows more packets to pass through. A threshold
value of 100percent means that no limit is placed on the traffic. A value of 0.0 means that all broadcast,
multicast, or unicast traffic on that port is blocked.
Note Because packets do not arrive at uniform intervals, the 1-second time i nte rval duri ng w hic h tra ffic
activity is measured can affect the behavior of storm control.
T
otal
n
umber of
b
roadcast
p
ackets
o
r bytes
Forwarded traff
ic
0T1
Threshold
T2 T4 T5
46651
T3 Time
Blocked traffic