1.1.c [i] Unicast flooding
Unicast flood is the unintentional behavior of a switch treating a unicast packet as a broadcast packet; a packet destined for one host is flooded or transmitted out of all the ports of a switch. The underlying cause of flooding is that destination MAC address of the packet is not in the L2 forwarding table (there is one for each VLAN) of the switch. The primary reasons for unicast flooding behavior include asymmetric routing, STP topology changes (i.e. repeated TCNs), and MAC forwarding table overflow.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 948-953). . Kindle Edition.