This 350-601 practice question tests your understanding of 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.
Two Nexus switches are configured for MACsec using MKA. The link between them is up, but MKA does not establish. Which issue is most likely?
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
✓
Mismatched cipher suites
MKA (MACsec Key Agreement) requires both endpoints to agree on a cipher suite during the CAK (Connectivity Association Key) exchange. If the cipher suites are mismatched (e.g., one switch uses GCM-AES-128 and the other uses GCM-AES-256), the MKA negotiation will fail, and the link will remain up but unsecured. This is the most common cause of MKA not establishing on an otherwise functional Layer 2 link.
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.
✗
Different MTU sizes
Why it's wrong here
MTU mismatch may cause packet drops but does not prevent MKA from initiating.
✓
Mismatched cipher suites
Why this is correct
MACsec requires the same cipher suite on both ends for MKA to succeed.
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.
✗
Switches are in different VLANs
Why it's wrong here
MACsec operates at Layer 2; VLAN membership does not affect MKA.
✗
Mismatched MKA key strings
Why it's wrong here
Key strings are used to generate the SAK but do not need to match across switches; each switch has its own keychain.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between MKA not establishing (cipher suite mismatch) versus MKA establishing but failing authentication (key string mismatch), leading candidates to incorrectly choose the key string option when the question specifically says 'does not establish'.
Detailed technical explanation
How to think about this question
MKA uses a four-way handshake (Distributed SAK) to derive session keys, and the cipher suite is negotiated in the first exchange (the SAK request/response). If the cipher suites do not match, the responder sends a failure notification and the initiator aborts. In real-world deployments, this often happens when one switch defaults to GCM-AES-128 (the IEEE 802.1AE-2006 default) and the other is configured for GCM-AES-256 (802.1AE-2018), or when one switch uses a proprietary cipher like GCM-AES-XPN.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched cipher suites — MKA (MACsec Key Agreement) requires both endpoints to agree on a cipher suite during the CAK (Connectivity Association Key) exchange. If the cipher suites are mismatched (e.g., one switch uses GCM-AES-128 and the other uses GCM-AES-256), the MKA negotiation will fail, and the link will remain up but unsecured. This is the most common cause of MKA not establishing on an otherwise functional Layer 2 link.
What should I do if I get this 350-601 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.
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Question Discussion
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