350-401 Question 127
Single answerA network engineer is troubleshooting a connectivity issue between two switches connected via an 802.1Q trunk link. The engineer observes that VLAN traffic is not passing between the switches as expected. Upon checking, the following configurations are found:
- Switch A's trunk port is configured with 'switchport trunk allowed vlan 10,20,30'.
- Switch B's trunk port is configured with 'switchport trunk allowed vlan 20,30,40'.
What could be the root cause of the issue?
- A
The native VLAN configuration is mismatched on both switches.
- B
VLAN 10 is not allowed on Switch B's trunk port.
- C
The trunking protocol is mismatched between Switch A and Switch B.
- D
802.1Q encapsulation is not enabled on Switch B.
Show answer and explanation
Correct answer: B
Explanation
The issue arises because the allowed VLAN list on Switch A includes VLAN 10, but Switch B does not permit VLAN 10 on its trunk port. For VLAN traffic to pass successfully, both switches must explicitly allow the VLAN on their trunk ports. Ensuring consistent 'switchport trunk allowed vlan' configurations on both ends resolves this issue.
- A. Incorrect.
A native VLAN mismatch can cause untagged traffic to be misinterpreted, but it does not prevent tagged VLANs (such as VLAN 20 and 30) from passing over the trunk link.
- B. Correct.
VLAN 10 is allowed on Switch A but not on Switch B, causing traffic for VLAN 10 to be dropped at Switch B. This is a common cause of VLAN traffic issues when trunk allowed VLAN lists are not consistent.
- C. Incorrect.
A trunking protocol mismatch (e.g., ISL vs. 802.1Q) could cause trunking failure, but there is no evidence from the scenario that this is the case. The issue is related to allowed VLANs.
- D. Incorrect.
802.1Q encapsulation is likely enabled since VLAN 20 and 30 are allowed on both switches. The issue is specifically with VLAN 10 being disallowed on Switch B.