7.1 Redundant Topologies  
  7.1.5 Multiple frame transmissions  
In a redundant switched network it is possible for an end device to receive multiple frames.

Assume that the MAC address of Router Y has been timed out by both switches. Also assume that Host X still has the MAC address of Router Y in its ARP cache and sends a unicast frame to Router Y. The router receives the frame because it is on the same segment as Host X.

Switch A does not have the MAC address of the Router Y and will therefore flood the frame out its ports. Switch B also does not know which port Router Y is on. Switch B then floods the frame it received causing Router Y to receive multiple copies of the same frame. This is a cause of unnecessary processing in all devices.