Chapter 87 Rate Limiting Overview - Dell PowerConnect J-EX4200-24T Software Manual

CHAPTER 87
Rate Limiting Overview
Understanding Storm Control on J-EX Series Switches
Understanding Storm Control on J-EX Series Switches on page 2511
Understanding Unknown Unicast Forwarding on J-EX Series Switches on page 2512
A traffic storm is generated when messages are broadcast on a network and each
message prompts a receiving node to respond by broadcasting its own messages on the
network. This, in turn, prompts further responses, creating a snowball effect. The LAN is
suddenly flooded with packets, creating unnecessary traffic that leads to poor network
performance or even a complete loss of network service. Storm control enables the
switch to monitor traffic levels and to drop broadcast and unknown unicast packets
when a specified traffic level—called the storm control level—is exceeded, thus preventing
packets from proliferating and degrading the LAN. As an alternative to having the switch
drop packets, you can configure it to shut down interfaces or temporarily disable interfaces
(see the
action-shutdown
control level is exceeded.
The factory default configuration enables storm control on all switch interfaces, with
the storm control level set to 80 percent of the combined broadcast and unknown unicast
streams. You can change the storm control level for an interface by specifying a bandwidth
value for the combined broadcast and unknown unicast traffic streams. You can also
selectively disable storm control on the broadcast stream or on the unknown unicast
stream.
Broadcast, multicast, and unicast packets are part of normal LAN operation, so to
recognize a storm, you must be able to identify when traffic has reached a level that is
abnormal for your LAN. Suspect a storm when operations begin timing out and network
response times slow down. As more packets flood the LAN, network users might be
unable to access servers or e-mail.
Monitor the level of broadcast and unknown unicast traffic in the LAN when it is operating
normally. Use this data as a benchmark to determine when traffic levels are too high.
Then configure storm control to set the level at which you want to drop broadcast traffic,
unknown unicast traffic, or both.
statement or the
port-error-disable
statement) when the storm
2511