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SSCP Practice Question: Is troubleshooting a site-to-site VPN that is…
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?
⚠ 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 blocked UDP 500 would prevent IKE negotiation starting, producing timeout or no response rather than 'no proposal chosen'. Firewall rules on port 500 are the right check when the tunnel cannot initiate at all, not when proposals fail to match.
- ✗
Incorrect peer IP address
Why it's wrong here
An incorrect peer IP address stops IKE packets reaching the responder, giving timeouts or no response, not a proposal mismatch. Verifying peer addresses is correct when the tunnel never initiates, whereas 'no proposal chosen' means IKEv2 parameters were exchanged but disagreed.
- ✓
Mismatched encryption algorithms
Why this is correct
IKEv2 'no proposal chosen' means the peers could not agree on a Phase 1 or Phase 2 proposal. Mismatched encryption or integrity algorithms between the two gateways is the classic cause, since the pre-shared key and version already match.
- ✗
Expired certificates
Why it's wrong here
'No proposal chosen' indicates IKE phase 1 could not agree encryption, hash, DH group or lifetime, so the tunnel never reaches certificate authentication. Certificates matter when a certificate-based peer identity is used, but here the pre-shared key is already confirmed correct.
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.
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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.