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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 50 Configuring Storm Control
About Storm Control
Note Storm control and Multicast storm control are supported in hardware on all ports on the WS-X4516,
WS-X4013+10GE, WS-X4516-10GE, WS-C4948, WS-C4948-10GE, WS-C4900M, WS-C4948E,
WS-X45-Sup6-E, and WS-X45-Sup6L-E. In contrast, the supervisor engines WS-X4515, WS-X4014,
and WS-X4013+ support storm control in hardware on non-blocking gigabit ports and in software on all
other ports, implying that the counters for these interfaces are approximate and computed.
Hardware-Based Storm Control Implementation
Broadcast suppression uses filtering that measures broadcast activity in a subnet over a one-second
interval and compares the measurement with a predefined threshold. If the threshold is reached, further
broadcast activity is suppressed for the duration of the interval. Broadcast suppression is disabled by
default.
Figure 50-1 shows the broadcast traffic patterns on a LAN interface over a given interval. In this
example, broadcast suppression occurs between times T1 and T2 and between T4 and T5. During those
intervals, the amount of broadcast traffic exceeded the configured threshold.
Figure 50-1 Storm Control Example—Hardware-based Implementation
The broadcast suppression threshold numbers and the time interval combination make the broadcast
suppression algorithm work with different levels of granularity. A higher threshold allows more
broadcast packets to pass through.
Broadcast suppression on the Catalyst 4500 series switches (Catalyst 4900M, Catalyst 4948E,
Supervisor Engine 6-E, and Supervisor Engine 6L-E) is implemented in hardware. The suppression
circuitry monitors packets passing from a LAN interface to the switching bus. If the packet destination
address is broadcast, then the broadcast suppression circuitry tracks the current count of broadcasts
within the one-second interval, and when a threshold is reached, it filters out subsequent broadcast
packets.
Because hardware broadcast suppression uses a bandwidth-based method to measure broadcast activity,
the most significant implementation factor is setting the percentage of total available bandwidth that can
be used by broadcast traffic. Because packets do not arrive at uniform intervals, the one-second interval
during which broadcast activity is measured can affect the behavior of broadcast suppression.
Tota l
number of
broadcast
packets
or bytes
0T1
Threshold
T2 T4 T5
S5706
T3 Time