This 350-401 practice question tests your understanding of infrastructure security. 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 implementing MACsec on a Cisco switch-to-switch link to provide encryption. Both switches support MACsec and are configured with the same pre-shared key (PSK). The engineer configures 'mka' and 'macsec' on the interfaces. After configuration, the link does not come up, and the engineer sees 'MKA not operational' in the show macsec status. 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 pre-shared key (PSK) configured on both switches does not match.
MACsec requires that both ends have matching keys and that the interfaces are in the same security mode (e.g., should-secure or must-secure). If one end is configured as 'must-secure' and the other as 'should-secure', they may not establish a secure channel. Option A is correct because a mismatch in the key chain or key string is a common issue. Option B is incorrect because MACsec can work with PSK. Option C is incorrect because MACsec does not require dot1q. Option D is incorrect because MACsec does not require a specific duplex setting.
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 pre-shared key (PSK) configured on both switches does not match.
Why this is correct
Correct because MKA requires matching keys to establish a secure channel.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
MACsec requires a RADIUS server for key distribution, which is not configured.
Why it's wrong here
Incorrect because MACsec can use PSK without a RADIUS server.
✗
The interfaces are configured with different VLANs, causing MACsec to fail.
Why it's wrong here
Incorrect because MACsec operates at Layer 2 and does not depend on VLAN configuration.
✗
The interfaces must be configured as trunk ports for MACsec to work.
Why it's wrong here
Incorrect because MACsec works on both access and trunk ports.
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.
Infrastructure Security — This question tests Infrastructure Security — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The pre-shared key (PSK) configured on both switches does not match. — MACsec requires that both ends have matching keys and that the interfaces are in the same security mode (e.g., should-secure or must-secure). If one end is configured as 'must-secure' and the other as 'should-secure', they may not establish a secure channel. Option A is correct because a mismatch in the key chain or key string is a common issue. Option B is incorrect because MACsec can work with PSK. Option C is incorrect because MACsec does not require dot1q. Option D is incorrect because MACsec does not require a specific duplex setting.
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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