Rockwell Automation Publication 1783-UM007G-EN-P - February 2017 309
Configure Switch Features Chapter 7
Port Thresholds
Port thresholds prevent traffic on a LAN from being disrupted by a broadcast,
multicast, or unicast storm on one of the physical interfaces. Port thresholds do
not apply to switches with lite firmware.
A LAN storm occurs when packets flood the LAN, creating excessive traffic
and degrading network performance. Errors in the protocol-stack
implementation, mistakes in network configurations, or users issuing denial-of-
service attacks can cause a storm.
Incoming (storm control)
Incoming port thresholds (or traffic suppression) monitor packets passing
from an interface to the switching bus and determines if the packet is unicast,
multicast, or broadcast. The switch counts the number of packets of a specified
type that is received within the 1-second time interval and compares the
measurement with a predefined suppression-level threshold.
Port thresholds use 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 remains blocked until the traffic rate drops below the falling
threshold and then resumes normal forwarding. In general, the higher the level,
the less effective the protection against broadcast storms.
The graph shows broadcast traffic patterns on an interface over a given 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 traffic exceeds the threshold, all traffic of that kind is
dropped for 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 not exceed the threshold, it is again forwarded.
IMPORTANT When the port threshold for multicast traffic is reached, all multicast traffic
is blocked. An exception is management traffic, such as bridge protocol data
unit (BDPU) and Cisco Discovery Protocol (CDP) frames.
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