According to this <a href=”http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/MACAddress.html”>Cisco site</a>, Frames that do not have a corresponding MAC address in the switches address table are handled like this:
The switch dynamically builds the address table by using the MAC source address of the frames received. When the switch receives a frame for a MAC destination address not listed in its address table, it floods the frame to all LAN ports of the same VLAN except the port that received the frame. When the destination station replies, the switch adds its relevant MAC source address and port ID to the address table. The switch then forwards subsequent frames to a single LAN port without flooding all LAN ports.
Note that other switches<b><i> may </i></b>handle this kind of situation differently.