The answer is that the voice VLAN is not configured on the interface. This is the most likely cause because an IP phone requires a dedicated voice VLAN to tag its frames with the correct VLAN ID for communication with the call manager; without the `switchport voice vlan` command, the phone cannot separate voice traffic from data traffic, preventing registration. Additionally, the PC connected to the phone’s PC port depends on the phone to forward untagged data traffic, and without a voice VLAN, the phone fails to negotiate properly, leaving the PC without an IP address. On the CCNA 200-301 v2 exam, this scenario tests your understanding of Cisco IP phone deployment and the critical difference between the native VLAN (for data) and the voice VLAN (for voice). A common trap is assuming that simply placing the interface in an access VLAN is enough, but the phone needs an explicit voice VLAN assignment to tag its packets. Remember the mnemonic: “Voice tags, data lags” — the voice VLAN tags traffic so the phone can register, while the data VLAN stays untagged for the PC.
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.
Exhibit
SW1# show running-config interface GigabitEthernet0/1
Building configuration...
Current configuration : 128 bytes
!
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
spanning-tree portfast
end
SW1#
A network administrator connects an IP phone to interface Gi0/1 and a PC to the phone's PC port. Users report that the phone cannot register with the call manager and the PC does not obtain an IP address. The administrator verifies the configuration shown in the exhibit. What is the most likely cause of the problem?
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.
SW1# show running-config interface GigabitEthernet0/1
Building configuration...
Current configuration : 128 bytes
!
interface GigabitEthernet0/1
switchport mode access
switchport access vlan 10
spanning-tree portfast
end
SW1#
A
The switchport mode should be changed to trunk to allow tagged voice traffic.
Why wrong: Trunk mode would forward tagged frames natively, but the PC's untagged traffic would be placed in the native VLAN, which would need to be set to the data VLAN. This is a valid but non‑standard workaround. The recommended and simpler solution is access mode with a voice VLAN.
B
The voice VLAN is not configured on the interface.
The exhibit lacks the 'switchport voice vlan <vlan-id>' command. Adding this command tells the switch to expect 802.1Q tagged frames for the phone's voice traffic while keeping the PC data in the access VLAN.
C
The spanning-tree portfast command is blocking the port from forwarding.
Why wrong: spanning-tree portfast sets the port to immediately transition to forwarding state upon link-up, bypassing the listening and learning phases. It does not block traffic.
D
The access VLAN 10 is incorrect for the data subnet.
Why wrong: The exhibit does not reveal the intended VLAN numbering, but the core symptom (phone cannot register) points to a voice VLAN issue. An incorrect data VLAN would affect the PC's IP assignment but not the phone's registration process.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The voice VLAN is not configured on the interface.
The correct answer is B because the voice VLAN is not configured on the interface. For an IP phone to communicate with the call manager, the switchport must be configured with a voice VLAN (e.g., 'switchport voice vlan 20') to separate voice traffic from data traffic. Without this command, the phone cannot tag its frames with the appropriate VLAN ID, preventing registration. Additionally, the PC connected to the phone's PC port relies on the phone to pass untagged data traffic, but without the voice VLAN, the phone may not properly negotiate or forward traffic, causing the PC to fail to obtain an IP address.
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 switchport mode should be changed to trunk to allow tagged voice traffic.
Why it's wrong here
Trunk mode would forward tagged frames natively, but the PC's untagged traffic would be placed in the native VLAN, which would need to be set to the data VLAN. This is a valid but non‑standard workaround. The recommended and simpler solution is access mode with a voice VLAN.
✓
The voice VLAN is not configured on the interface.
Why this is correct
The exhibit lacks the 'switchport voice vlan <vlan-id>' command. Adding this command tells the switch to expect 802.1Q tagged frames for the phone's voice traffic while keeping the PC data in the access VLAN.
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 spanning-tree portfast command is blocking the port from forwarding.
Why it's wrong here
spanning-tree portfast sets the port to immediately transition to forwarding state upon link-up, bypassing the listening and learning phases. It does not block traffic.
✗
The access VLAN 10 is incorrect for the data subnet.
Why it's wrong here
The exhibit does not reveal the intended VLAN numbering, but the core symptom (phone cannot register) points to a voice VLAN issue. An incorrect data VLAN would affect the PC's IP assignment but not the phone's registration process.
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.
✓The voice VLAN is not configured on the interface.Correct answer▾
Why this is correct
The exhibit lacks the 'switchport voice vlan <vlan-id>' command. Adding this command tells the switch to expect 802.1Q tagged frames for the phone's voice traffic while keeping the PC data in the access VLAN.
✗The switchport mode should be changed to trunk to allow tagged voice traffic.Wrong answer — click to see why▾
Why this is wrong here
Cisco best practice for IP phone + PC is an access port with switchport voice vlan, not trunk mode. Trunk mode is more complex and can cause VLAN hopping if misconfigured.
✗The spanning-tree portfast command is blocking the port from forwarding.Wrong answer — click to see why▾
Why this is wrong here
Portfast enables faster convergence, not a blocking action. A blocked port would show no link or constant amber LED, not intermittent connectivity.
✗The access VLAN 10 is incorrect for the data subnet.Wrong answer — click to see why▾
Why this is wrong here
Even if VLAN 10 were wrong, the phone’s registration issue would persist because the voice VLAN is still missing. The problem has two symptoms; the voice VLAN omission explains both.
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: answer the scenario, not the keyword
Cisco often tests the distinction between access VLAN and voice VLAN, and the trap here is that candidates assume the access VLAN alone is sufficient for both voice and data, overlooking the mandatory 'switchport voice vlan' command required for IP phone operation.
Detailed technical explanation
How to think about this question
Cisco IP phones use CDP (Cisco Discovery Protocol) or LLDP to negotiate the voice VLAN with the switch. When 'switchport voice vlan' is configured, the switch tells the phone to tag its voice frames with that VLAN ID, while the PC connected to the phone's PC port continues to send untagged frames on the access VLAN. Without this configuration, the phone may default to using the access VLAN for voice traffic, which can cause registration failures if the call manager is on a different subnet, and the phone may not properly forward DHCP requests from the PC, leading to IP address assignment failures.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The voice VLAN is not configured on the interface. — The correct answer is B because the voice VLAN is not configured on the interface. For an IP phone to communicate with the call manager, the switchport must be configured with a voice VLAN (e.g., 'switchport voice vlan 20') to separate voice traffic from data traffic. Without this command, the phone cannot tag its frames with the appropriate VLAN ID, preventing registration. Additionally, the PC connected to the phone's PC port relies on the phone to pass untagged data traffic, but without the voice VLAN, the phone may not properly negotiate or forward traffic, causing the PC to fail to obtain an IP address.
What should I do if I get this 200-301 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.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. A network administrator connects an IP phone to interface GigabitEthernet1/0/1 on a Cisco Catalyst switch. The PC connected through the phone works normally on the data VLAN, but the phone cannot obtain an IP address and fails to register with the call server. Which action will resolve the problem?
hard
A.Configure 'switchport trunk allowed vlan 10,20' on interface Gi1/0/1 and change the switchport mode to trunk.
✓ B.Add 'switchport voice vlan 20' on interface GigabitEthernet1/0/1.
C.Issue 'switchport access vlan 20' and 'switchport voice vlan 10' on interface Gi1/0/1.
D.Enable 'switchport port-security mac-address sticky' on the interface and manually add the phone’s MAC address to the access VLAN.
Why B: The PC works on the data VLAN, but the phone cannot obtain an IP address, indicating the phone is not receiving the correct VLAN assignment. The phone should be placed in the voice VLAN (VLAN 20) using the 'switchport voice vlan 20' command, which allows the switch to tag the phone's traffic with VLAN 20 via CDP/LLDP, enabling it to get an IP from the voice VLAN DHCP server.
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Question Discussion
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