Control Network Newsletter

October 2009: Broadcast Storm Control

Broadcast messages are those sent to all possible destinations on a network. An Ethernet switch is designed to differentiate traffic by target destination – thereby achieving more efficient communication than is possible with a repeating hub. But if broadcast messages are sent when a switch is in the network, the switching strategy is defeated and the switch behaves much like a repeating hub.

A broadcast message is one with a destination MAC ID value of all ones (FF FF FF FF FF FF). Too many of such messages can be problematic, but Ethernet switches have a feature called Broadcast Storm Control that can be employed to mitigate the issue. However, under some circumstances, Broadcast Storm Control can itself cause problems.

Broadcast Storm Control is disabled on all unmanaged (plug-and-play) switches from Contemporary Controls. This policy was adopted because some users will suffer dropped messages if they employ an Ethernet strategy that sends continuous broadcast messages. Instead of sending directed messages that use the essential characteristic of a switch, some applications deliberately broadcast messages to every device – even though only one may need it. Since Broadcast Storm Control is seldom needed by most users, Contemporary Controls made a conscious decision in October 2008 to disable this feature on all unmanaged products.

Contemporary Controls offers a configurable switch (the EISC Series) in which Broadcast Storm Control can, at the user's discretion, be enabled or disabled globally (applying to all ports or to none). On managed switches, Broadcast Storm Control is not explicitly available. Instead, a similar strategy is accomplished with a feature known as Rate Control which allows the user to control rates of various types of traffic and vary the control by port. Like unmanaged switches, our managed switches have this feature disabled by default – but it can be enabled by the user quite easily.

If a customer wishes, unmanaged switches from Contemporary Controls can have Broadcast Storm Control enabled at the factory – by request and at no charge. When enabled, each switch port will drop broadcast packets after receiving a continuous stream of 64 broadcast packets. The counter will reset to 0 every 800 ms or if it receives a non-broadcast packet (one whose destination MAC ID is something other than FF FF FF FF FF FF).

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