- A
The interface needs the 'authentication guest-vlan <vlan-id>' command to specify the VLAN for non-802.1X devices.
Correct because the guest VLAN is a separate configuration that tells the switch to place the port into a specific VLAN when authentication fails or times out.
- B
The switch must have 'aaa authentication dot1x default group radius' configured globally.
Why wrong: Incorrect because the scenario implies that authentication is already configured globally; the issue is specific to the interface behavior after failure.
- C
The 'authentication host-mode multi-host' command should be replaced with 'authentication host-mode multi-domain' to support guest VLAN.
Why wrong: Incorrect because 'multi-host' mode allows multiple hosts on the port and does not prevent guest VLAN usage; the missing piece is the guest VLAN definition.
- D
The port must be configured as a trunk port to allow the guest VLAN.
Why wrong: Incorrect because the guest VLAN is a single VLAN assigned to the access port; trunking is not required.
Quick Answer
The answer is that the interface is missing the 'authentication guest-vlan <vlan-id>' command. Without this explicit configuration, a port set to 'authentication port-control auto' will remain in the unauthorized state when a non-802.1X-capable device connects, because the switch has no defined fallback VLAN for authentication failures. The guest VLAN feature is specifically designed to grant limited network access to devices that either fail authentication or are not 802.1X-capable, but it must be manually assigned per interface. On the ENCOR 350-401 exam, this scenario tests your understanding of the 802.1X state machine and the difference between port-control modes and VLAN assignment—a common trap is assuming 'auto' alone triggers a guest VLAN. Remember the mnemonic: "Auto gets you started, but Guest VLAN gets them admitted."
CCNP AAA, RADIUS, and TACACS+ Practice Question
This 350-401 practice question tests your understanding of aaa, radius, and tacacs+. 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 Cisco switch for 802.1X port-based authentication. The switch is configured with a RADIUS server for authentication. The engineer wants to allow devices that fail 802.1X authentication to still access a limited guest VLAN. The engineer configures 'authentication port-control auto' and 'authentication host-mode multi-host' on the interface. However, when a non-802.1X-capable device is connected, the port remains in the unauthorized state and does not fall into the guest VLAN. What is missing?
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
The interface needs the 'authentication guest-vlan <vlan-id>' command to specify the VLAN for non-802.1X devices.
For a port to move to a guest VLAN when authentication fails, the switch must be configured with a guest VLAN on that interface. The 'authentication port-control auto' enables 802.1X, but without a guest VLAN defined, the port stays unauthorized on failure.
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 interface needs the 'authentication guest-vlan <vlan-id>' command to specify the VLAN for non-802.1X devices.
Why this is correct
Correct because the guest VLAN is a separate configuration that tells the switch to place the port into a specific VLAN when authentication fails or times out.
Related concept
Authentication checks who the user is.
- ✗
The switch must have 'aaa authentication dot1x default group radius' configured globally.
Why it's wrong here
Incorrect because the scenario implies that authentication is already configured globally; the issue is specific to the interface behavior after failure.
- ✗
The 'authentication host-mode multi-host' command should be replaced with 'authentication host-mode multi-domain' to support guest VLAN.
Why it's wrong here
Incorrect because 'multi-host' mode allows multiple hosts on the port and does not prevent guest VLAN usage; the missing piece is the guest VLAN definition.
- ✗
The port must be configured as a trunk port to allow the guest VLAN.
Why it's wrong here
Incorrect because the guest VLAN is a single VLAN assigned to the access port; trunking is not required.
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.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario implies that authentication is already configured globally; the issue is specific to the interface behavior after failure.
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.
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FAQ
Questions learners often ask
What does this 350-401 question test?
AAA, RADIUS, and TACACS+ — This question tests AAA, RADIUS, and TACACS+ — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The interface needs the 'authentication guest-vlan <vlan-id>' command to specify the VLAN for non-802.1X devices. — For a port to move to a guest VLAN when authentication fails, the switch must be configured with a guest VLAN on that interface. The 'authentication port-control auto' enables 802.1X, but without a guest VLAN defined, the port stays unauthorized on failure.
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.
What is the key concept behind this question?
Authentication checks who the user is.
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Last reviewed: Jun 18, 2026
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