The correct configuration step is to configure the switch port as a trunk port. This is required because the router-on-a-stick design relies on 802.1Q trunking to carry multiple VLANs over a single physical link, with the router’s subinterfaces each handling a different VLAN ID. Without the switch port set to trunk mode, frames would not be tagged with VLAN IDs, and the router’s subinterfaces would be unable to distinguish between guest and corporate traffic, breaking inter-VLAN routing. On the ENCOR 350-401 exam, this concept tests your understanding of Layer 2 trunking and subinterface termination, often appearing in a scenario where a candidate mistakenly configures the port as an access port or forgets to set the native VLAN. A common trap is assuming the router interface alone handles tagging, but the switch port must explicitly be set to trunk mode to forward tagged frames. Remember the mnemonic: “Trunk to the router, or the subinterfaces won’t matter.”
CCNP Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 small business has a single router connected to the internet and a switch for the LAN. They want to implement VLANs to separate guest and corporate traffic. The router has only one physical interface to the switch. The network engineer proposes using subinterfaces with 802.1Q trunking on the router interface. Which configuration step is required on the switch port connected to the router?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the port as a trunk port.
The router uses subinterfaces with 802.1Q trunking to carry multiple VLANs over a single physical link. For this to work, the switch port connected to the router must be configured as a trunk port, which tags frames with VLANIDs as they traverse the link. This allows the router to route between VLANs using its subinterfaces, each associated with a specific VLAN.
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.
✗
Configure the port as a routed port.
Why it's wrong here
A routed port is a Layer 3 interface and does not support VLAN tagging.
✗
Configure the port as an access port in VLAN 1.
Why it's wrong here
An access port carries only one VLAN, which would not support multiple VLANs to the router.
✓
Configure the port as a trunk port.
Why this is correct
A trunk port allows multiple VLANs via 802.1Q tagging, enabling the router subinterfaces to work.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure the port as a dynamic desirable port.
Why it's wrong here
Dynamic desirable uses DTP to negotiate trunking; but it's not the required configuration; the port must be a trunk.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a switch port connecting to a router can remain as an access port or use DTP, but the key is that the router's subinterface requires 802.1Q-tagged frames, which only a statically configured trunk port can provide.
Detailed technical explanation
How to think about this question
802.1Q trunking inserts a 4-byte VLAN tag into the Ethernet frame header, allowing multiple VLANs to share a single link. On the router, subinterfaces are configured with encapsulation dot1Q <vlan-id> to match the tag, and the switch port must be set to trunk mode (switchport mode trunk) to preserve these tags. A common real-world scenario is a router-on-a-stick design, where this configuration enables inter-VLAN routing without a Layer 3 switch, but the switch port must explicitly allow the required VLANs with the switchport trunk allowed vlan command.
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.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the port as a trunk port. — The router uses subinterfaces with 802.1Q trunking to carry multiple VLANs over a single physical link. For this to work, the switch port connected to the router must be configured as a trunk port, which tags frames with VLAN IDs as they traverse the link. This allows the router to route between VLANs using its subinterfaces, each associated with a specific VLAN.
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.
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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