N10-009 Question 4
Single answerLayer 2 - Data linkA network administrator is troubleshooting intermittent connectivity between two departments connected through managed switches. Users in both departments can reach servers on their own VLANs, but devices in VLAN 20 on one switch cannot consistently communicate with devices in VLAN 20 on another switch. Devices in VLAN 10 work normally across the same uplink. The administrator confirms that routing is functioning and that the uplink between switches is physically up. Which of the following is the MOST likely cause?
- A
The trunk link between the switches is not permitting VLAN 20
- B
The default gateway for VLAN 20 is configured with the wrong subnet mask
- C
A DNS server for VLAN 20 is unreachable
- D
The switches are using different STP bridge priorities
Show answer and explanation
Correct answer: A
Explanation
This scenario focuses on a Layer 2 data link issue involving VLAN tagging and trunking between switches. When one VLAN works across a switch uplink and another does not, the best first suspicion is trunk misconfiguration, especially the allowed VLAN list or VLAN tagging mismatch. On IEEE 802.1Q trunks, each VLAN must be permitted across the trunk for its frames to traverse the link. Because VLAN 10 is functioning, the physical link and basic trunk operation are likely intact. Because routing is confirmed to be working, Layer 3 causes are less likely. Network+ candidates should recognize this as a practical troubleshooting pattern: isolate whether the problem is physical, Layer 2, or Layer 3, then compare working and non-working VLANs. Best practices from enterprise switch documentation commonly recommend verifying trunk status, native VLAN settings, and allowed VLAN lists when troubleshooting inter-switch VLAN communication.
- A. Correct.
Correct. This is a classic Layer 2 trunking issue. If VLAN 10 traffic passes across the inter-switch uplink but VLAN 20 traffic does not, the most likely explanation is that VLAN 20 is missing from the allowed VLAN list on the trunk or the trunk is otherwise not carrying that VLAN. This would prevent frames tagged for VLAN 20 from traversing the uplink while allowing other VLANs, such as VLAN 10, to work normally.
- B. Incorrect.
Incorrect. A wrong subnet mask on the default gateway is a Layer 3 configuration problem. The scenario specifically indicates that routing has already been verified and that the issue affects communication for the same VLAN across switches, which points more directly to a Layer 2 transport problem than to IP gateway configuration.
- C. Incorrect.
Incorrect. DNS resolution problems affect name-to-IP lookups, not whether devices on the same VLAN can communicate at Layer 2 across a trunk. Even if DNS were unavailable, direct communication by IP would still be possible if the VLAN were correctly carried between switches.
- D. Incorrect.
Incorrect. Spanning Tree Protocol bridge priority differences are normal in switched environments and are used to influence root bridge selection. Different priorities by themselves would not specifically prevent only VLAN 20 from traversing the uplink while VLAN 10 continues to function. If STP were blocking the only path, the impact would more likely be broader or tied to topology changes rather than one missing VLAN.