350-401 Question 125
Single answerAn engineer is troubleshooting a connectivity issue between two Cisco switches connected via an 802.1Q trunk. The engineer notices that VLAN traffic is not passing properly across the trunk. Upon investigation, the following configurations are observed:
Switch A:
- Interface is configured with 'switchport mode dynamic desirable'
- Native VLAN is set to 10
Switch B:
- Interface is configured with 'switchport mode trunk'
- Native VLAN is set to 20
What could be the cause of the issue?
- A
The native VLAN mismatch between the two switches.
- B
The 'switchport mode trunk' command on Switch B is incorrect.
- C
Dynamic desirable mode on Switch A does not establish a trunk with Switch B.
- D
802.1Q encapsulation is not enabled on the trunk interfaces.
Show answer and explanation
Correct answer: A
Explanation
The issue in this scenario is the native VLAN mismatch between Switch A and Switch B. When the native VLANs are different, untagged traffic sent from one switch will not be properly understood by the other, leading to connectivity issues. The other configurations, such as the trunk mode and dynamic desirable mode, are valid and do not cause the problem.
- A. Correct.
A native VLAN mismatch between the two switches can cause traffic to be dropped or misrouted, as each switch sends untagged traffic to different VLANs.
- B. Incorrect.
The 'switchport mode trunk' command on Switch B is correct and allows the interface to actively operate as a trunk.
- C. Incorrect.
Dynamic desirable mode on Switch A is compatible with 'switchport mode trunk' on Switch B, as the trunk will form successfully.
- D. Incorrect.
802.1Q encapsulation is automatically enabled when trunking is configured on modern Cisco switches, so this is not the issue in this scenario.