NSE7 IKEv2 phase1 proposal Practice Question
A FortiGate administrator is troubleshooting a VPN tunnel that uses IKEv2 with certificate authentication. The tunnel fails to establish, and the IKE debug shows 'no acceptable proposal' for the initial exchange. Which TWO configuration mismatches could cause this error? (Choose two.)
⚠ Common exam trap
The error 'no acceptable proposal' occurs during IKEv2 phase1 (SA_INIT), not phase2. Phase2 mismatches cause 'no acceptable proposal' at a different stage.
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 between the two peers
'No acceptable proposal' in IKEv2 phase1 indicates a mismatch in proposal parameters. Encryption algorithm mismatch (A) and authentication method mismatch (E: pre-shared key vs certificate) can both cause this error. Phase2 mismatches (B) occur later, not during initial exchange.
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 encryption algorithms between the two peers
Why this is correct
IKEv2 phase1 encryption must match; e.g., AES256 vs AES128.
- ✗
Mismatched phase2 encryption algorithms
Why it's wrong here
Phase2 mismatch would cause error after phase1 is established.
- ✗
Mismatched IKE version (IKEv1 vs IKEv2)
Why it's wrong here
IKE version mismatch would cause a different error, typically 'no acceptable proposal' as well? Actually, IKEv1 and IKEv2 are different; but if both are configured for IKEv2, version is not the issue. However, if one uses IKEv1 and the other IKEv2, that could cause proposal mismatch. But the question specifies IKEv2, so version mismatch is less likely.
- ✗
Incorrect local certificate configuration on one peer
Why it's wrong here
Incorrect certificate would cause authentication failure, not proposal mismatch.
- ✓
Mismatched authentication methods (pre-shared key vs certificate)
Why this is correct
If one peer expects pre-shared key and the other expects certificate, the proposal fails.
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
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