Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 65 Configuring Storm Control
Enabling Broadcast Storm Control
Figure 65-1 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 65-1 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 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.
Software-Based Storm Control Implementation
When storm control is enabled on an interface, the switch monitors packets received on the interface and
determines whether the packets are broadcast. The switch monitors the number of broadcast packets
received within a one-second time interval. When the interface threshold is met, all incoming data traffic
on the interface is dropped. This threshold is specified as a percentage of total available bandwidth that
can be used by broadcast traffic. If the lower threshold is specified, all data traffic is forwarded as soon
as the incoming traffic falls below that threshold.
Enabling Broadcast Storm Control
To enable storm control, perform this task:
Total
number of
broadcast
packets
or bytes
0 T1
Threshold
T2 T4 T5
S5706
T3 Time