- A
Verify the DHCP scope for VLAN 10 on the DHCP server.
Why wrong: While a depleted DHCP scope could prevent address assignment, the more likely immediate cause given the symptoms is the phone's internal VLAN handling. The technician should first rule out the phone's passthrough configuration before investigating DHCP server issues.
- B
Verify that CDP is enabled on the IP phone.
Why wrong: CDP is used for voice VLAN discovery and is likely already functioning because the phone has successfully registered and is operational. CDP does not control the data VLAN for the attached PC; therefore, it is not relevant to this issue.
- C
Verify the QoS trust state on the switch port.
Why wrong: QoS trust states affect the marking and prioritization of packets, not the basic VLAN assignment or IP address assignment. Misconfigured QoS would not prevent a PC from obtaining an IP address.
- D
Verify the IP phone's passthrough mode for the PC port.
The IP phone acts as a switch; if the phone's PC port is not configured to pass traffic untagged on the correct VLAN (passthrough mode), the PC's frames will be dropped or placed in the wrong VLAN. Checking this setting directly addresses the path from PC to switch.
Quick Answer
The answer is to verify the IP phone's passthrough mode for the PC port. This is correct because the switch port is already properly configured with an access VLAN 10 for data and a voice VLAN 100 for the phone’s traffic, and the phone itself is functional, so the issue lies in how the phone handles the PC’s untagged frames. In passthrough mode, the phone’s internal switch should bridge the PC traffic onto the access VLAN without tagging, but if the phone is set to a different mode—like tagging the PC traffic or isolating the port—the PC will fail to obtain an IP address. On the CCNA 200-301 v2 exam, this tests your understanding of IP phone integration and the subtle distinction between phone and PC VLAN handling, a common trap where candidates focus on switchport configuration rather than the phone’s internal settings. Remember the memory tip: “Phone works, PC stuck? Check the passthrough luck.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. 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.
An IP phone connected to switch port Gi0/4 is working and receiving calls, but the PC connected to the phone's data port cannot obtain an IP address. The technician confirms that interface Gi0/4 has switchport mode access and shows switchport access vlan 10 and switchport voice vlan 100. What should the technician do next?
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
Verify the IP phone's passthrough mode for the PC port.
The switch port is correctly configured with the appropriate access VLAN (10) for data and voice VLAN (100). Since the phone itself is functional, the issue is likely in how the phone tags or forwards the PC's untagged frames. The phone's internal switch must be set to passthrough mode to bridge the PC traffic onto the access VLAN. Verifying this setting will identify if the phone is misconfigured, causing the PC to be placed in the wrong VLAN or isolated.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Verify the DHCP scope for VLAN 10 on the DHCP server.
- ✗
Verify that CDP is enabled on the IP phone.
Why it's wrong here
CDP is used for voice VLAN discovery and is likely already functioning because the phone has successfully registered and is operational. CDP does not control the data VLAN for the attached PC; therefore, it is not relevant to this issue.
- ✗
Verify the QoS trust state on the switch port.
- ✓
Verify the IP phone's passthrough mode for the PC port.
Why this is correct
The IP phone acts as a switch; if the phone's PC port is not configured to pass traffic untagged on the correct VLAN (passthrough mode), the PC's frames will be dropped or placed in the wrong VLAN. Checking this setting directly addresses the path from PC to switch.
Related concept
Access ports place end devices into a single VLAN.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Verify the IP phone's passthrough mode for the PC port.Correct answer▾
Why this is correct
The IP phone acts as a switch; if the phone's PC port is not configured to pass traffic untagged on the correct VLAN (passthrough mode), the PC's frames will be dropped or placed in the wrong VLAN. Checking this setting directly addresses the path from PC to switch.
✗Verify the DHCP scope for VLAN 10 on the DHCP server.Wrong answer — click to see why▾
Why this is wrong here
Assumes the switch port configuration alone guarantees proper VLAN delivery to the PC, ignoring the phone's role as a transparent bridge.
✗Verify that CDP is enabled on the IP phone.Wrong answer — click to see why▾
Why this is wrong here
Confuses the mechanism for voice VLAN assignment with the requirement for data passthrough; CDP's role is only for the phone's own voice VLAN, not for the PC's data VLAN.
✗Verify the QoS trust state on the switch port.Wrong answer — click to see why▾
Why this is wrong here
Misapplies QoS as a potential cause for a connectivity issue; it is a quality-of-service feature and does not block DHCP or initial network access.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Verify the IP phone's passthrough mode for the PC port. — The switch port is correctly configured with the appropriate access VLAN (10) for data and voice VLAN (100). Since the phone itself is functional, the issue is likely in how the phone tags or forwards the PC's untagged frames. The phone's internal switch must be set to passthrough mode to bridge the PC traffic onto the access VLAN. Verifying this setting will identify if the phone is misconfigured, causing the PC to be placed in the wrong VLAN or isolated.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 14, 2026
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