If you’ve ever tried to get advanced Cisco certifications, you’ve probably encountered questions dealing with the mismatch between the end device ARP timeouts and the L2 switch CAM (MAC address cache) timeouts. If you’re still wondering what the underlying problem is (it took me a while to figure it out), read the Unicast Flooding in Switched Campus Networks document from Cisco.
In all scenarios, traffic sent to unknown unicast MAC address causes layer-2 flooding, which can significantly reduce switch performance. Microsoft took this problem to a completely new level with its Network Load Balancing implementation: Windows servers send ARP replies containing MAC address X from MAC address Y, causing all the traffic toward the servers to be flooded. Dragan has encountered this problem in a large customer network and described his “experience” in Fragments.