The answer is mismatched encryption algorithms. The 'no proposal chosen' error in IPsec VPN occurs during the IKE_SA_INIT exchange when two peers cannot agree on a common set of security parameters, such as encryption algorithms, hash algorithms, or Diffie-Hellman groups, even when the pre-shared key and IKE version are correct. On the Systems Security Certified Practitioner SSCP exam, this scenario tests your understanding of IKEv2 negotiation phases and the importance of matching proposal sets across both sides of a site-to-site VPN. A common trap is assuming the pre-shared key is the only authentication factor, but the error specifically points to a mismatch in the cryptographic suite. Remember the memory tip: "No proposal chosen" means "no common ground"—check your ciphers, hashes, and DH groups first, not your keys.
SSCP Network and Communications Security Practice Question
This SSCP practice question tests your understanding of network and communications 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 troubleshooting a site-to-site VPN that is failing to establish. The pre-shared key is correct and both sides use IKEv2. The VPN logs show 'no proposal chosen'. 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.
Why wrong: A firewall block would prevent any IKE traffic, typically resulting in timeouts rather than proposal errors.
B
Incorrect peer IP address
Why wrong: An incorrect peer IP would typically result in a timeout or no response, not a proposal mismatch error.
C
Mismatched encryption algorithms
IKE negotiations require matching proposals; a mismatch in encryption, integrity, or DH group causes 'no proposal chosen'.
D
Expired certificates
Why wrong: Since a pre-shared key is used, certificates are not involved; expired certificates would be an authentication failure, not proposal mismatch.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Mismatched encryption algorithms
The 'no proposal chosen' error in IKEv2 indicates that the two VPN peers cannot agree on a common set of security parameters during the IKE_SA_INIT exchange. Since the pre-shared key is correct and both sides use IKEv2, the most likely cause is a mismatch in the encryption algorithms (e.g., AES-256 vs. AES-128), hash algorithms (e.g., SHA-256 vs. SHA-1), or Diffie-Hellman groups (e.g., group 14 vs. group 2). This prevents the IKE security association from being established.
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.
✗
Firewall blocking UDP port 500
Why it's wrong here
A firewall block would prevent any IKE traffic, typically resulting in timeouts rather than proposal errors.
✗
Incorrect peer IP address
Why it's wrong here
An incorrect peer IP would typically result in a timeout or no response, not a proposal mismatch error.
✓
Mismatched encryption algorithms
Why this is correct
IKE negotiations require matching proposals; a mismatch in encryption, integrity, or DH group causes 'no proposal chosen'.
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.
✗
Expired certificates
Why it's wrong here
Since a pre-shared key is used, certificates are not involved; expired certificates would be an authentication failure, not proposal mismatch.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between 'no proposal chosen' (which points to a cryptographic parameter mismatch) and 'no response' or 'timeout' (which points to connectivity or firewall issues), leading candidates to incorrectly select firewall blocking when the error message clearly indicates a proposal negotiation failure.
Detailed technical explanation
How to think about this question
IKEv2 uses a two-phase process: IKE_SA_INIT (phase 1) negotiates cryptographic parameters and generates keys, while IKE_AUTH (phase 2) authenticates the peers. The 'no proposal chosen' error occurs specifically during IKE_SA_INIT when the initiator's proposal set (e.g., transform types like ENCR, PRF, INTEG, DH groups) does not match any of the responder's configured policies. In real-world troubleshooting, you can use 'debug crypto ikev2' on Cisco devices to see the exact proposals sent and received, revealing which parameter is mismatched.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
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.
Network and Communications Security — This question tests Network and Communications Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched encryption algorithms — The 'no proposal chosen' error in IKEv2 indicates that the two VPN peers cannot agree on a common set of security parameters during the IKE_SA_INIT exchange. Since the pre-shared key is correct and both sides use IKEv2, the most likely cause is a mismatch in the encryption algorithms (e.g., AES-256 vs. AES-128), hash algorithms (e.g., SHA-256 vs. SHA-1), or Diffie-Hellman groups (e.g., group 14 vs. group 2). This prevents the IKE security association from being established.
What should I do if I get this SSCP 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. A network engineer is configuring a site-to-site VPN. The remote peer is using AES-256 encryption and SHA-1 for integrity. Which configuration parameter is likely misconfigured?
hard
A.Transform-set
B.Pre-shared key
✓ C.Group 5
D.IKE lifetime
Why C: The remote peer is using AES-256 encryption and SHA-1 for integrity, which are both supported in an IPsec transform-set. However, the exhibit shows that the local peer is configured with Group 5 (Diffie-Hellman 1536-bit), while the remote peer likely expects a different DH group (e.g., Group 2 or Group 14). DH group mismatch during Phase 1 (IKE) negotiation causes the VPN tunnel to fail, even if encryption and integrity algorithms match.
Last reviewed: Jun 30, 2026
Question Discussion
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This SSCP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SSCP exam.
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