The answer is a mismatched encryption or authentication algorithm within the transform set. This is the most likely cause because the debug output “IPSEC(validate_proposal): transform set proposal mismatch” explicitly indicates that the IPsec proposals exchanged during phase 2 do not align, even when the transform set names appear identical on both peers. A subtle difference—such as AES-256 on one side versus AES-128 on the other, or SHA-1 versus SHA-256—will trigger this failure, while IKE phase 1 completing successfully rules out pre-shared key or routing issues. On the Cisco SCOR 350-701 exam, this question tests your ability to distinguish between phase 1 and phase 2 failures and to recognize that transform set parameters must match exactly, not just the set name. A common trap is assuming identical configuration lines guarantee a match, but the algorithms themselves must be byte-for-byte identical. Memory tip: “Phase 2 is picky—encryption and hash must match exactly, not just the set name.”
350-701 Security Concepts Practice Question
This 350-701 practice question tests your understanding of security concepts. 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 troubleshooting a site-to-site IPsec VPN that fails to establish. The IKE phase 1 completes successfully, but phase 2 fails. The debug output shows 'IPSEC(validate_proposal): transform set proposal mismatch'. Both peers have the same transform set configured. 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
✓
Mismatched encryption/authentication algorithms in the transform set
The error 'IPSEC(validate_proposal): transform set proposal mismatch' indicates that the IPsec transform sets proposed by the two peers do not match during IKE phase 2 negotiation. Even if the transform sets appear identical in configuration, a mismatch in the encryption algorithm (e.g., AES-256 vs AES-128) or authentication algorithm (e.g., SHA-1 vs SHA-256) will cause this failure. Since IKE phase 1 completed successfully, the pre-shared key and routing are not the issue, and lifetime mismatches typically generate a different error.
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.
✗
Mismatched IPsec lifetime values
Why it's wrong here
Can cause mismatch but debug shows transform set, not lifetime.
✗
Missing route for the remote subnet
Why it's wrong here
Would cause traffic not to trigger VPN, not a transform set mismatch.
✓
Mismatched encryption/authentication algorithms in the transform set
Why this is correct
Even if both sets are named the same, the actual algorithms might differ; 'transform set proposal mismatch' indicates algorithm mismatch.
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.
✗
Incorrect pre-shared key
Why it's wrong here
PSK mismatch would fail phase 1, not phase 2.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between IKE phase 1 and phase 2 failures, and the trap here is that candidates assume identical transform set names mean identical algorithms, ignoring that default values (like AES key length) can differ between devices or IOS versions.
Trap categories for this question
Command / output trap
Can cause mismatch but debug shows transform set, not lifetime.
Detailed technical explanation
How to think about this question
In IPsec, the transform set defines the encryption and authentication algorithms (e.g., esp-aes 256 esp-sha-hmac) used for protecting data in phase 2. The IKE daemon compares the received proposal against the local transform set; if any algorithm differs—even if the names appear similar (e.g., esp-aes vs esp-aes 256)—the proposal is rejected. A common real-world scenario is when one peer uses 'esp-aes' (defaulting to AES-128) while the other explicitly configures 'esp-aes 256', causing a mismatch that is easily overlooked.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 Concepts — This question tests Security Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched encryption/authentication algorithms in the transform set — The error 'IPSEC(validate_proposal): transform set proposal mismatch' indicates that the IPsec transform sets proposed by the two peers do not match during IKE phase 2 negotiation. Even if the transform sets appear identical in configuration, a mismatch in the encryption algorithm (e.g., AES-256 vs AES-128) or authentication algorithm (e.g., SHA-1 vs SHA-256) will cause this failure. Since IKE phase 1 completed successfully, the pre-shared key and routing are not the issue, and lifetime mismatches typically generate a different error.
What should I do if I get this 350-701 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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