NSE7 Troubleshooting and Diagnostics Practice Question
An admin is troubleshooting an IPsec VPN tunnel that is failing phase 2. The IKE debug shows 'no matching proposal'. Which TWO settings should the admin verify on both sides? (Choose two.)
⚠ Common exam trap
A common mix-up: candidates confuse phase 1 and phase 2 parameters, incorrectly selecting pre-shared key (option D) or gateway IPs (option E) as causes for a phase 2 proposal mismatch, when in fact only the IPsec SA parameters (encryption, authentication, PFS) are negotiated in phase 2.
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
✓
Encryption algorithm (e.g., AES128, AES256)
In IPsec phase 2, the IKE debug message 'no matching proposal' indicates a mismatch in the security association (SA) parameters used to establish the IPsec SA. The encryption algorithm (option B) is a core component of the IPsec proposal that must match exactly on both peers. Perfect Forward Secrecy (PFS) using a Diffie-Hellman group (option C) is also negotiated during phase 2; if one side requires PFS and the other does not, or if the DH groups differ, phase 2 will fail with this error.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Dead Peer Detection interval
Why it's wrong here
DPD intervals govern liveness detection of the peer during phase 1 and established tunnels, not proposal matching in phase 2. It is tempting because DPD misconfiguration does break tunnels, and it would be the correct choice when a tunnel drops silently after establishment.
- ✓
Encryption algorithm (e.g., AES128, AES256)
Why this is correct
Phase 2 (Quick Mode) negotiates the IPsec SA using the Phase 2 proposal, so a mismatch in the encryption algorithm between peers produces exactly the "no matching proposal" error. Verifying AES128 or AES256 on both gateways ensures the Phase 2 encryption transforms align, satisfying the stem's requirement to resolve the failing negotiation.
- ✓
Diffie-Hellman group for PFS
Why this is correct
The phase 2 'no matching proposal' error occurs when the PFS Diffie-Hellman group differs between peers, since phase 2 rekeys derive fresh keying material using that group. Verifying the DH group on both gateways satisfies the stem's requirement, as mismatched groups (for example group 5 versus group 14) prevent the quick mode proposal from matching.
- ✗
Pre-shared key
Why it's wrong here
A pre-shared key mismatch fails phase 1 authentication, producing an IKE SA failure rather than a phase 2 'no matching proposal'. It is tempting because PSK errors are a common VPN fault, and verifying it would be correct when phase 1 authentication fails.
- ✗
Local and remote gateway IP addresses
Why it's wrong here
Gateway IP addresses are negotiated in phase 1 to build the IKE SA; a phase 2 'no matching proposal' indicates mismatched proxy IDs, encryption, or authentication algorithms. It is tempting because address mismatches do cause tunnel failures, but those surface during phase 1.
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
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This NSE7 practice question is part of Courseiva's free Fortinet 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 NSE7 exam.