A Cisco switch will do one of the three things with
an incoming frame;
• Forward it
• Flood it
• Filter it
To make this decision, the switch consults its MAC
Address table to check if there’s an entry for the
destination MAC address – but first, the switch will
actually check to see if there’s an entry for the
source MAC address of the frame, because it’s that
source MAC that the switch will use to actually build
the table in the first place.
To illustrate the concept, look at this illustration;
continue reading: http://tutorialslodge.com/building-mac-table/