This 350-401 practice question tests your understanding of 802.1x and trustsec. 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 802.1X on a Cisco Catalyst 9300 switch for a wired network. The engineer wants to allow devices that do not support 802.1X (e.g., printers) to still access the network using MAB (MAC Authentication Bypass). The engineer configures the interface with 'authentication port-control auto', 'dot1x pae authenticator', and 'mab'. However, after connecting a printer, the switch logs show 'MAB failed' repeatedly. The printer's MAC address is in the RADIUS server database. 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 RADIUS server is not configured to accept MAC addresses in the format sent by the switch (e.g., with dots or colons).
MAB requires the switch to send a MAC address as the username and password. If the RADIUS server does not accept the format, authentication fails. Option A is correct because the RADIUS server must be configured to accept MAC addresses in the format sent by the switch (e.g., 'aaaa.bbbb.cccc'). Option B is incorrect because MAB does not require EAP. Option C is incorrect because the switchport mode does not affect MAB. Option D is incorrect because the printer does not support 802.1X, so it cannot respond to EAP.
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 RADIUS server is not configured to accept MAC addresses in the format sent by the switch (e.g., with dots or colons).
Why this is correct
Correct because MAB uses the MAC address as credentials; format mismatch causes failure.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
The switch is not configured with 'dot1x timeout tx-period' to initiate MAB.
Why it's wrong here
Incorrect because MAB is initiated by the switch after 802.1X times out; no separate timer is needed for MAB.
✗
The interface is configured as 'switchport mode trunk', which does not support MAB.
Why it's wrong here
Incorrect because MAB works on trunk ports as well.
✗
The printer is not responding to EAP-Request/Identity packets.
Why it's wrong here
Incorrect because MAB is used for non-802.1X devices; the switch does not expect EAP responses.
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 RADIUS server is not configured to accept MAC addresses in the format sent by the switch (e.g., with dots or colons). — MAB requires the switch to send a MAC address as the username and password. If the RADIUS server does not accept the format, authentication fails. Option A is correct because the RADIUS server must be configured to accept MAC addresses in the format sent by the switch (e.g., 'aaaa.bbbb.cccc'). Option B is incorrect because MAB does not require EAP. Option C is incorrect because the switchport mode does not affect MAB. Option D is incorrect because the printer does not support 802.1X, so it cannot respond to EAP.
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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