200-301 Question 152
Select 3A network engineer is troubleshooting a connectivity issue in a switched network. They observe that a host connected to a switch port is unable to communicate with other devices. Upon examining the MAC address table of the switch, the MAC address of the host is not present. Which of the following actions could explain why the MAC address is missing from the MAC address table?
- A
The switch has not yet received a frame from the host.
- B
The MAC address entry for the host has aged out due to inactivity.
- C
The host is using an unsupported MAC address format.
- D
The switch port connected to the host is configured as an access port.
- E
The switch port connected to the host is in an error-disabled state.
Show answer and explanation
Correct answers: A, B, E
Explanation
Switches learn MAC addresses by examining the source MAC address of frames received on their ports. If the switch has not yet received any frames from the host, or if the MAC address entry has aged out due to inactivity, the MAC address will not be in the table. Additionally, if the port is in an error-disabled state, it cannot process traffic or learn MAC addresses. These are the most likely reasons for the observed behavior.
- A. Correct.
If the switch has not yet received any frames from the host, it will not learn the MAC address, and it will not appear in the MAC address table.
- B. Correct.
The MAC address table uses an aging timer to remove inactive MAC addresses. If the host has not sent traffic for a while, its entry may have been removed.
- C. Incorrect.
Switches can learn any valid MAC address format. Unsupported MAC address formats are highly unlikely to be the cause of the issue.
- D. Incorrect.
Configuring a port as an access port does not prevent the switch from learning the MAC address of the connected device. This is not relevant to the problem.
- E. Correct.
If the port is in an error-disabled state, it will not forward traffic or learn the MAC address of the connected host.