The answer is that the Nexus switch is missing the 'switchport trunk encapsulation dot1q' command. This is required because while the Cisco Nexus 9000 defaults to 802.1Q encapsulation, the Catalyst 3850 expects an explicit encapsulation statement to be configured on the trunk interface; without it, the trunk fails to negotiate properly. On the ENCOR 350-401 exam, this tests your understanding of platform-specific trunking defaults—a common trap where engineers assume all Cisco switches handle encapsulation the same way. The Catalyst 3850 does not support ISL, but it still requires the explicit dot1q keyword on the Nexus side to bring the trunk up. A useful memory tip: "Nexus needs the dot1q nod" — think of the Nexus switch needing a verbal nod (explicit command) to confirm encapsulation, even when it’s already using the correct standard.
CCNP VLANs and Trunking Practice Question
This 350-401 practice question tests your understanding of vlans and trunking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is configuring a new Cisco Nexus 9000 switch to connect to an existing Cisco Catalyst 3850 switch. The link between them should be a trunk carrying VLANs 10, 20, and 30. The engineer configures the Nexus switch with 'switchport mode trunk' and 'switchport trunk allowed vlan 10,20,30'. However, the trunk does not come up. The Catalyst switch is configured with 'switchport mode trunk' and 'switchport trunk allowed vlan 10,20,30'. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Nexus switch does not have the 'switchport trunk encapsulation dot1q' command configured.
The most likely cause is that the Nexus switch defaults to 802.1Q encapsulation, but the Cisco Catalyst 3850 switch requires the explicit 'switchport trunk encapsulation dot1q' command to be configured on the Nexus side. Without this command, the Nexus switch may not properly negotiate or establish the trunk, as the Catalyst switch expects a specific encapsulation type. This is a common issue when interconnecting different Cisco switch platforms that handle trunk encapsulation defaults differently.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The Nexus switch does not have the 'switchport trunk encapsulation dot1q' command configured.
Why this is correct
Correct because some Nexus switches require explicit encapsulation configuration for trunking to work with Catalyst switches.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Catalyst switch is set to dynamic auto mode.
Why it's wrong here
Incorrect because the scenario states both are set to trunk mode.
✗
The allowed VLAN list on the Nexus switch is missing VLAN 1.
Why it's wrong here
Incorrect because VLAN 1 is not required for trunking; it is allowed by default.
✗
The native VLAN is set to 999 on the Nexus switch.
Why it's wrong here
Incorrect because if the native VLAN were different, the trunk would still form, but native VLAN traffic would be problematic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that Nexus switches do not require the 'switchport trunk encapsulation dot1q' command because they only support 802.1Q, but the command is still necessary for trunk formation with certain Catalyst switches.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario states both are set to trunk mode.
Detailed technical explanation
How to think about this question
On Cisco Nexus switches, the default trunk encapsulation is 802.1Q, but the 'switchport trunk encapsulation dot1q' command is still required to explicitly set the encapsulation type when connecting to certain older or non-Nexus switches that do not auto-negotiate encapsulation. This command is mandatory on Catalyst switches that support both ISL and 802.1Q, but on Nexus switches, it is often overlooked because the switch only supports 802.1Q. In real-world scenarios, failing to include this command can cause the trunk to remain in a down state, especially when the remote switch expects an explicit encapsulation negotiation.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
VLANs and Trunking — This question tests VLANs and Trunking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Nexus switch does not have the 'switchport trunk encapsulation dot1q' command configured. — The most likely cause is that the Nexus switch defaults to 802.1Q encapsulation, but the Cisco Catalyst 3850 switch requires the explicit 'switchport trunk encapsulation dot1q' command to be configured on the Nexus side. Without this command, the Nexus switch may not properly negotiate or establish the trunk, as the Catalyst switch expects a specific encapsulation type. This is a common issue when interconnecting different Cisco switch platforms that handle trunk encapsulation defaults differently.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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