mediumMultiple ChoiceObjective-mapped
Fixing 'No Proposal Chosen' IPsec VPN Error
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?
Quick Answer
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.
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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'.
- ✗
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.
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
Go deeper
Related to this question
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Same concept, more angles
1 more way this is tested on SSCP
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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.