Allied Telesis AT-8100 Series - Manual (Page 456)

Chapter 25: Link-flap Protection
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Overview
A port that is unable to maintain a reliable connection to a network node
may experience a condition referred to as link-flapping. This problem,
which is usually caused by intermittent problems with network cables or
network nodes, causes the state of a link on a port to fluctuate up and
down.
A fluctuating link can disrupt more than the connectivity of a single port.
Other switch operations may be affected as well. If, for instance, a
fluctuating link is part of a spanning tree domain or a member of an LACP
trunk, the switch attempts to compensate by redirecting traffic away from
the link when it is down and to the link when it is up. Frequent traffic
redistributions such as this are an inefficient use of the switch’s resources
and can result in the additional loss of traffic.
Link-flap protection minimizes the disruption to your network from this type
of problem. It stabilizes the network topology by automatically disabling
ports that experience link-flap events. A port that is disabled due to link-
flap events remains disabled until you enable it again with the
management software, such as with the standard NO SHUTDOWN
command or the LINK-FLAP PROTECTION command. The switch notifies
you of link-flap events by entering messages in the event logs and
transmitting SNMP traps.
You define the rate and duration that constitute link-flap events. These
values are set at the switch level. The rate defines the number of link
changes that have to occur to signal a link-flap event. A link change is
defined as any time a port loses a link or establishes a link to an end node.
When a port establishes a link to a network node, that represents one link
change. And when a port loses a link, that is another link change. The rate
has a range of 4 to 65,535 changes.
The duration is the time period in which the changes must occur. It has a
range of 20 to 65,535 seconds.
The default values are ten changes for the rate and 60 seconds for the
duration. At these settings, a link-flap event is signaled when a port
experiences ten link changes in one minute. If, as an example, you set the
rate to five changes and the duration to 120 seconds, a link-flap event
occurs when a port’s link changes five times within two minutes.
While the rate and the duration are set at the switch level, link-flap
protection is activated at the port level. This means you can activate it on
just those ports where you believe the problem is most likely to occur or
that are connected to devices that are critical to the functioning of your
network. This feature requires only minimal processing by the switch and
can be activated on all of the switch’s ports without affecting network
performance.