The answer is that the IP phone does not support 802.1X and is not configured for MAC Authentication Bypass (MAB). In a voice VLAN deployment, the switch must authenticate the phone separately from the PC; if the phone lacks 802.1X supplicant capability, it cannot complete the EAP exchange, and without MAB configured as a fallback, the switch never assigns the phone to the voice VLAN. This scenario tests your understanding of multi-domain authentication on Cisco switches, a common troubleshooting point in the ENCOR 350-401 exam where the trap is assuming the phone’s VLAN is automatically granted by the voice VLAN configuration alone. Remember, the phone must authenticate—either via 802.1X or MAB—before the switch places it into the voice VLAN; the PC’s successful authentication does not influence the phone’s VLAN assignment. A useful memory tip: “Phone first, then voice—no auth, no choice.”
CCNP 802.1X and TrustSec Practice Question
This 350-401 practice question tests your understanding of 802.1x and trustsec. 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 network engineer is configuring 802.1X on a Cisco switch for a voice VLAN deployment. The switchport is connected to an IP phone, which then connects to a PC. The engineer configures the interface with 'authentication port-control auto', 'dot1x pae authenticator', and 'switchport voice vlan 10'. The PC authenticates successfully, but the IP phone does not get an IP address from the voice VLAN. The engineer verifies that the phone is configured for 802.1X and the RADIUS server is correct. 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 IP phone does not support 802.1X and is not configured for MAB.
In a voice VLAN deployment, the switch must be configured to authenticate the phone separately from the PC. The phone typically uses 802.1X or MAB. If the phone does not authenticate, it may be placed in the data VLAN or denied. Option C is correct because the phone must authenticate to be placed in the voice VLAN. Option A is incorrect because the phone can use MAB. Option B is incorrect because the voice VLAN is configured. Option D is incorrect because the PC's authentication does not affect the phone's VLAN.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 IP phone does not support 802.1X and is not configured for MAB.
Why this is correct
Correct because the phone must authenticate to be placed in the voice VLAN; if it fails, it may not get the voice VLAN.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
The switchport is missing 'switchport mode access' command.
Why it's wrong here
Incorrect because the voice VLAN can work with access or trunk ports.
✗
The RADIUS server is not sending the voice VLAN ID in the Access-Accept.
Why it's wrong here
Incorrect because the voice VLAN is configured locally on the switch; RADIUS can override it but is not required.
✗
The PC is using the voice VLAN instead of the data VLAN.
Why it's wrong here
Incorrect because the PC authenticates separately and should be in the data VLAN.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-401 questions on access control and AAA configuration.
802.1X and TrustSec — This question tests 802.1X and TrustSec — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The IP phone does not support 802.1X and is not configured for MAB. — In a voice VLAN deployment, the switch must be configured to authenticate the phone separately from the PC. The phone typically uses 802.1X or MAB. If the phone does not authenticate, it may be placed in the data VLAN or denied. Option C is correct because the phone must authenticate to be placed in the voice VLAN. Option A is incorrect because the phone can use MAB. Option B is incorrect because the voice VLAN is configured. Option D is incorrect because the PC's authentication does not affect the phone's VLAN.
What should I do if I get this 350-401 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-401 questions on access control and AAA configuration.
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?
Authentication checks who the user is.
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