Multicasts are treated as broadcasts by the switches. Broadcasts and multicasts frames are flooded out all ports, except the one on which the frame was received.

If Host X sends a broadcast, like an ARP request for the Layer 2 address of the router, then Switch A will forward the broadcast out all ports. Switch B, being on the same segment, also forwards all broadcasts. Switch B sees all the broadcasts that Switch A forwarded and Switch A sees all the broadcasts that Switch B forwarded. Switch A sees the broadcasts and forwards them. Switch B sees the broadcasts and forwards them.

The switches continue to propagate broadcast traffic over and over. This is called a broadcast storm. This broadcast storm will continue until one of the switches is disconnected. The switches and end devices will be so busy processing the broadcasts that user traffic is unlikely to flow. The network will appear to be down or extremely slow.

Web Links Shining the Lights on Broadcast Storms http://www.networkuptime.com/tips/lights/