This 350-401 practice question tests your understanding of vlans and trunking. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer runs the following command on Switch SW1:
SW1# show interfaces gi0/1 trunk
Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1
Port Vlans allowed on trunk Gi0/1 10,20
Port Vlans allowed and active in management domain Gi0/1 10,20
Port Vlans in spanning tree forwarding state and not pruned Gi0/1 10,20
Based on this output, what can be concluded?
VLAN 1 is allowed on this trunk.
Why wrong: The allowed list shows only 10,20; VLAN 1 is not included.
The trunk is using DTP dynamic desirable mode.
Why wrong: Mode is 'on', which is static trunking without DTP.
Only VLANs 10 and 20 are allowed on this trunk.
The 'Vlans allowed on trunk' line explicitly shows 10,20.
The native VLAN is 10.
Why wrong: Native VLAN is 1.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Only VLANs 10 and 20 are allowed on this trunk.
The output shows that the 'Vlans allowed on trunk' list contains only VLANs 10 and 20. This means the trunk has been explicitly configured to permit only those VLANs, and all other VLANs (including VLAN 1) are pruned or blocked from traversing the trunk. Therefore, only VLANs 10 and 20 are allowed, making option C correct.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
VLAN 1 is allowed on this trunk.
Why it's wrong here
The allowed list shows only 10,20; VLAN 1 is not included.
The trunk is using DTP dynamic desirable mode.
Why it's wrong here
Mode is 'on', which is static trunking without DTP.
Only VLANs 10 and 20 are allowed on this trunk.
Why this is correct
The 'Vlans allowed on trunk' line explicitly shows 10,20.
Related concept
Read the scenario before looking for a memorised answer.
The native VLAN is 10.
Why it's wrong here
Native VLAN is 1.
Common exam traps
Cisco often tests the distinction between the native VLAN and the allowed VLAN list; candidates mistakenly assume that the native VLAN is always permitted on the trunk, but the allowed list explicitly controls which VLANs can pass traffic, and the native VLAN must be included in that list to be forwarded.
Trap categories for this question
Command / output trap
The allowed list shows only 10,20; VLAN 1 is not included.
Detailed technical explanation
The 'switchport trunk allowed vlan' command controls which VLANs are permitted to traverse a trunk link. Even though the native VLAN (VLAN 1 by default) is often assumed to be allowed, it is not automatically included unless explicitly added to the allowed list. In real-world scenarios, restricting allowed VLANs is a security best practice to prevent VLAN hopping and to limit broadcast domains across trunks.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
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FAQ
VLANs and Trunking — This question tests VLANs and Trunking — Read the scenario before looking for a memorised answer..
The correct answer is: Only VLANs 10 and 20 are allowed on this trunk. — The output shows that the 'Vlans allowed on trunk' list contains only VLANs 10 and 20. This means the trunk has been explicitly configured to permit only those VLANs, and all other VLANs (including VLAN 1) are pruned or blocked from traversing the trunk. Therefore, only VLANs 10 and 20 are allowed, making option C correct.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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