switch floods packets with unknown destination MAC addresses to all ports

By default, a switch floods packets with unknown destination MAC addresses to all ports. If unknown unicast and multicast traffic is forwarded to a switch port, there might be security issues. To prevent forwarding such traffic, you can configure a port to block unknown unicast or multicast packets

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat4000/12_1_20/config/uniflood.pdf