The answer is a hash algorithm mismatch, because the IKEv1 policy hash algorithm was not explicitly configured and defaults may differ between routers. In IPsec VPN negotiation, Phase 1 requires all IKE policy parameters—including the hash algorithm—to match exactly on both peers. When an engineer configures an IKEv1 policy without specifying the hash, Cisco IOS defaults to MD5 on some versions and SHA-1 on others, causing the IKEv1 policy hash algorithm mismatch that prevents the tunnel from establishing. On the ENCOR 350-401 exam, this scenario tests your understanding that “identical” configurations can still fail due to hidden defaults, a common trap where candidates focus only on encryption, group, and lifetime. The strongest algorithms require explicit configuration of SHA-256 or SHA-512. Memory tip: “Hash it out or it won’t match”—always verify the hash algorithm is set identically on both sides.
CCNP VPN Technologies Practice Question
This 350-401 practice question tests your understanding of vpn technologies. 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 site-to-site IPsec VPN between two Cisco routers. The engineer wants to ensure that the VPN tunnel uses the strongest possible encryption and authentication algorithms. The engineer configures the following: crypto isakmp policy 10, authentication pre-share, encryption aes-256, group 14, lifetime 86400. On the remote router, the engineer configures: crypto isakmp policy 10, authentication pre-share, encryption aes-256, group 14, lifetime 86400. The tunnel fails to establish. 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 hash algorithm is not specified and defaults may differ between routers.
The IKE policy parameters must match exactly on both peers. In this scenario, the policies appear identical, but a common oversight is that the hash algorithm (e.g., SHA-256) is not specified in the policy; the default is MD5 or SHA-1 depending on IOS version. If one router uses default SHA-1 and the other uses MD5, the mismatch will prevent Phase 1 from completing. Option B is correct because the hash algorithm mismatch is a frequent cause of failure. Option A is incorrect because the lifetimes match. Option C is incorrect because group 14 is valid. Option D is incorrect because pre-shared keys can be used with strong encryption.
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.
✗
The lifetimes are set too high; they should be 3600 seconds.
Why it's wrong here
The lifetimes match on both routers, so this is not the cause.
✓
The hash algorithm is not specified and defaults may differ between routers.
Why this is correct
Correct because the default hash algorithm can vary, causing a 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.
✗
The Diffie-Hellman group 14 is not supported on these routers.
Why it's wrong here
Group 14 is widely supported on modern Cisco routers.
✗
Pre-shared keys cannot be used with AES-256 encryption.
Why it's wrong here
Pre-shared keys are compatible with AES-256.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
VPN Technologies — This question tests VPN Technologies — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The hash algorithm is not specified and defaults may differ between routers. — The IKE policy parameters must match exactly on both peers. In this scenario, the policies appear identical, but a common oversight is that the hash algorithm (e.g., SHA-256) is not specified in the policy; the default is MD5 or SHA-1 depending on IOS version. If one router uses default SHA-1 and the other uses MD5, the mismatch will prevent Phase 1 from completing. Option B is correct because the hash algorithm mismatch is a frequent cause of failure. Option A is incorrect because the lifetimes match. Option C is incorrect because group 14 is valid. Option D is incorrect because pre-shared keys can be used with strong encryption.
What should I do if I get this 350-401 question wrong?
Identify which 350-401 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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