200-301 Question 164
Select 4A network administrator is troubleshooting connectivity issues on a switch in a Layer 2 network. They examine the MAC address table and notice a specific MAC address is missing from the table. What could be a possible reason for this?
- A
The switch has not received any frames from the device associated with that MAC address.
- B
The MAC address aging timer has expired for that entry.
- C
The MAC address belongs to a device in a different VLAN.
- D
The switch is in a state where it has disabled all ports.
- E
The MAC address table has reached its maximum capacity.
Show answer and explanation
Correct answers: A, B, C, E
Explanation
A MAC address may be missing from the MAC address table if the switch has not learned the MAC address due to inactivity, aging timer expiration, VLAN segmentation, or a full MAC address table. Understanding these scenarios is crucial for diagnosing connectivity issues in a Layer 2 network.
- A. Correct.
Correct: If the switch has not received any frames from the device, it cannot dynamically learn the MAC address and will not add it to the MAC address table.
- B. Correct.
Correct: Switches have an aging timer for MAC address table entries. If no frames are received from the device within the aging time, the MAC address entry will be removed.
- C. Correct.
Correct: VLANs segment the switch's Layer 2 domain. A MAC address from a different VLAN will not appear in the MAC address table of another VLAN.
- D. Incorrect.
Incorrect: While a switch could disable its ports due to issues like spanning-tree blocking or errors, this state does not directly explain why a MAC address is missing from the table.
- E. Correct.
Correct: If the MAC address table is full, new MAC addresses cannot be added, which may prevent the missing MAC address from appearing.