200-301 Question 151
Single answerA network engineer is troubleshooting a Layer 2 switch in a network. The switch is configured to dynamically populate its MAC address table. The engineer notices that some MAC addresses are being removed from the table after a certain period of time. What could be the cause of this behavior?
- A
The switch has reached the maximum limit of its MAC address table capacity.
- B
The MAC address aging timer has expired for those entries.
- C
The devices associated with the removed MAC addresses are powered off.
- D
The switch has been rebooted and cleared the MAC address table.
Show answer and explanation
Correct answer: B
Explanation
Switches dynamically learn MAC addresses by examining the source MAC address of incoming frames. Each MAC address entry is associated with an aging timer. If no traffic is seen from a MAC address within the aging time, the entry is removed to optimize memory usage. This prevents outdated or unused MAC addresses from occupying space in the MAC address table.
- A. Incorrect.
This option is incorrect because when the MAC address table reaches its maximum limit, it does not remove entries randomly. Instead, the switch typically uses an algorithm to handle new entries, such as overwriting older or less-used entries. This behavior is unrelated to the aging timer.
- B. Correct.
This is the correct answer. Switches use an aging timer for MAC addresses. If no traffic is received from a specific MAC address within the aging timer period, the entry is removed from the MAC address table to free up space.
- C. Incorrect.
This option is incorrect. While powered-off devices stop sending traffic, their MAC addresses are not immediately removed unless the aging timer expires. The removal is tied to the aging timer, not the devices being powered off.
- D. Incorrect.
This option is incorrect because while rebooting a switch does clear the MAC address table, this behavior is not related to the aging timer. Rebooting the switch is a manual action, not an automatic process like MAC address aging.