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350-401 Practice Question: An engineer is troubleshooting a problem where a…
An engineer is troubleshooting a problem where a trunk link between two Cisco switches is not passing traffic for VLAN 10, but other VLANs are working. The trunk is configured with switchport mode trunk on both sides. The engineer checks the allowed VLAN list and sees VLAN 10 is included. The native VLAN is set to 1 on both sides. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between a VLAN being allowed on a trunk and a VLAN being created in the VLAN database, leading candidates to focus on trunk configuration rather than verifying the VLAN's existence on both switches.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 10 is not created in the VLAN database on one of the switches.
The most likely cause is that VLAN 10 is not created in the VLAN database on one of the switches. Even if VLAN 10 is included in the allowed VLAN list on the trunk, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database. The trunk interface will be operationally down for that specific VLAN, preventing traffic from passing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VLAN 10 is not created in the VLAN database on one of the switches.
Why this is correct
VLAN 10 must exist in the local VLAN database on both switches before traffic can cross an 802.1Q trunk. If one switch lacks VLAN 10, that switch will not forward tagged frames for that VLAN, even though the trunk interface is administratively up and the VLAN is in the allowed list. The switch will not create the VLAN dynamically unless VTP is in server mode and advertises it, so the missing database entry is the root cause.
- ✗
VTP pruning has removed VLAN 10 from the trunk.
Why it's wrong here
VTP pruning would remove VLAN 10 from the trunk's allowed list only when no active access port exists for that VLAN on the downstream switch, and this would be visible with show interface trunk. If VLAN 10 still appears in the allowed list on the trunk, pruning has not occurred; the failure is due to the VLAN not being defined in the VLAN database on one switch. VTP pruning cannot remove a VLAN from the database itself—it only controls which VLANs are permitted on specific trunk links.
- ✗
The native VLAN is mismatched.
Why it's wrong here
A native VLAN mismatch appears on an 802.1Q trunk when the two ends are configured with different untagged VLAN IDs. This condition triggers CDP error messages and can put the trunk into an inconsistent state, but it affects only untagged traffic on the native VLAN, not a tagged VLAN such as VLAN 10. Since the problem is specific to VLAN 10 and other VLANs are presumably passing, native VLAN mismatch is not a valid explanation.
- ✗
Spanning Tree Protocol is blocking VLAN 10 on the trunk.
Why it's wrong here
While STP can block a VLAN on a trunk in PVST+ or Rapid PVST+, it does so only when a loop or redundant path exists for that VLAN. A single trunk link without an alternative path is not placed in a blocking state for one VLAN, and the port would remain forwarding for all other VLANs. Additionally, STP blocking would not hide a missing VLAN database entry—it would still show VLAN 10 as defined but in a discarding state, which is not the situation described.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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