How does a switch learn MAC addresses and what commonly causes CAM table overflow?

How does a switch learn MAC addresses and what commonly causes CAM table overflow?

• A. The switch uses a hash table unrelated to frames; aging occurs every hour.
• B. CAM tables are fixed in size and do not overflow.
• C. The switch learns MACs from destination addresses; entries age after inactivity; heavy endpoint churn or virtualization can cause overflow.
• D. The switch learns MACs from source addresses; entries age after inactivity; heavy endpoint churn or virtualization can cause overflow.

Answer: (D)

A switch learns MAC addresses by watching the source MAC address on frames as they arrive on each port. When a frame comes in, the switch notes the sender’s MAC and the port it came from, and it adds that mapping to the MAC (CAM) table so future frames destined for that MAC can be forwarded directly to the correct port. Destination addresses are used to decide where to forward frames once a mapping exists, not to learn new addresses. Entries in the CAM table age out after a period of inactivity, making space for new devices. If devices keep appearing or there are many virtual NICs and VM migrations (heavy endpoint churn or virtualization), the number of unique MACs the switch must track can exceed the CAM table’s capacity, causing overflow. When overflow happens, the switch can’t learn new addresses and may flood unknown unicast frames until entries age out again. So this option correctly reflects learning from source addresses, aging after inactivity, and overflow caused by rapid churn or virtualization.

A switch learns MAC addresses by watching the source MAC address on frames as they arrive on each port. When a frame comes in, the switch notes the sender’s MAC and the port it came from, and it adds that mapping to the MAC (CAM) table so future frames destined for that MAC can be forwarded directly to the correct port. Destination addresses are used to decide where to forward frames once a mapping exists, not to learn new addresses.

Entries in the CAM table age out after a period of inactivity, making space for new devices. If devices keep appearing or there are many virtual NICs and VM migrations (heavy endpoint churn or virtualization), the number of unique MACs the switch must track can exceed the CAM table’s capacity, causing overflow. When overflow happens, the switch can’t learn new addresses and may flood unknown unicast frames until entries age out again.

So this option correctly reflects learning from source addresses, aging after inactivity, and overflow caused by rapid churn or virtualization.