Under which circumstance will a switch drop a frame?

The only time a frame is dropped is when the cyclic redundancy check (CRC) calculated against the frame's payload deems the frame corrupt. If the destination MAC address is not known, it will be flooded to all active ports on the switch, but it will not be dropped. If the source MAC address of the frame is unknown in the MAC address table, the source MAC address will be learned on the incoming port. If the destination MAC address exists in another switch's MAC address table, then it will either be forwarded to that switch or flooded to that switch if the current switch has an empty MAC address table for the entry.