200-301 Question 197
Single answerA network administrator is configuring a trunk link between two switches using 802.1Q. The administrator notices that traffic from VLAN 20 is not passing between the switches, while traffic from other VLANs is unaffected. What could be the most likely cause of this issue?
- A
VLAN 20 is not allowed on the trunk.
- B
The native VLAN on both switches is set to VLAN 20.
- C
802.1Q encapsulation is not enabled on the trunk interface.
- D
The trunk link is using the ISL protocol instead of 802.1Q.
Show answer and explanation
Correct answer: A
Explanation
When configuring a trunk link using 802.1Q, you must explicitly allow specific VLANs on the trunk if you want their traffic to pass. By default, some switches block certain VLANs unless they are manually allowed. Since other VLANs are passing traffic, it indicates that 802.1Q is working and the issue is specific to VLAN 20 being disallowed on the trunk.
- A. Correct.
If VLAN 20 is not explicitly allowed on the trunk, traffic from this VLAN will not pass, even though the trunk link is functional for other VLANs.
- B. Incorrect.
Setting the native VLAN to VLAN 20 on both sides would not prevent traffic for VLAN 20 from passing over the trunk, as the native VLAN is untagged and only affects traffic handling for untagged frames.
- C. Incorrect.
If 802.1Q encapsulation were not enabled, no VLAN traffic would pass through the trunk, not just VLAN 20. The fact that other VLANs are working indicates 802.1Q is already enabled.
- D. Incorrect.
The question specifies the trunk is configured using 802.1Q. ISL is an alternative encapsulation protocol, but it is not relevant in this scenario.