The CAM table cannot be populated with the source address so other action must be taken. Is the switch going to drop the packet?
A switch dynamically builds the MAC address table by examining the source MAC address of the frames received on a port. The switch forwards frames by searching for a match between the destination MAC address in the frame and an entry in the MAC address table. CAM simply refers to the way the switch uses memory (in a content-addresable) manner to look up the MAC address to port association.
The following process is performed on every Ethernet frame that enters a switch:
- Learn: Examining the Source MAC Address. Every frame that enters a switch is checked for new information to learn. It does this by examining the frame’s source MAC address and port number where the frame entered the switch. If the source MAC address does not exist, it is added to the table along with the incoming port number. If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes.
- Forward: Examining the Destination MAC Address. Next, if the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. If the destination MAC address is in the table, it will forward the frame out the specified port. If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is known as an unknown unicast. If the destination MAC address is a broadcast or a multicast, the frame is also flooded out all ports except the incoming port.