NSE4 Authentication and VPN Practice Question
A network administrator is troubleshooting an IPsec VPN tunnel between two FortiGates. The tunnel is established, but traffic is not passing. The administrator runs 'diagnose vpn tunnel list' and sees the tunnel is up. Which two additional diagnostics should the administrator run to isolate the issue?
⚠ Common exam trap
A common pitfall is to assume that 'diagnose debug application ike -1' is only useful during initial tunnel negotiation and not after the tunnel is up. In reality, this debug can be invaluable for troubleshooting issues like phase 2 rekey failures, SA mismatches, or traffic selector problems that occur after the tunnel is established. Another trap is to overlook the session table diagnostics (Option A), which directly show whether expected traffic is hitting the firewall policies and whether sessions are being created or dropped. Both tools are needed for a complete analysis.
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
✓
diagnose sys session filter and diagnose sys session list
'diagnose sys session filter' allows you to set filters (e.g., by source/destination IP or port) and 'diagnose sys session list' then displays the current session table entries matching those filters. Since the IPsec tunnel is up but traffic is not passing, the session table may show that traffic is being dropped or not matching the expected policies, helping isolate whether the issue is with policy routing, firewall rules, or session setup. Option C is also correct because 'diagnose debug application ike -1' enables detailed IKE debugging that can reveal post-establishment issues, such as rekey failures, phase 2 problems, or IPsec SA mismatches, which may not be apparent from the tunnel list alone. Both diagnostics are appropriate: A checks the data plane (session table), and C checks the control plane (IKE negotiation details).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
diagnose sys session filter and diagnose sys session list
Why this is correct
This allows checking if sessions are being created and whether they match the expected policy.
- ✗
diagnose hardware sysinfo memory
Why it's wrong here
Memory info is not relevant to VPN traffic flow.
- ✓
diagnose debug application ike -1
Why this is correct
This shows IKE negotiations and can reveal Phase 2 mismatches or policy drops.
- ✗
diagnose netlink interface list
Why it's wrong here
Interface status is already known; tunnel is up.
- ✗
execute ping-options source and execute ping
Why it's wrong here
Ping tests connectivity but does not isolate configuration issues.
Visual reference
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 NSE4 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 NSE4 exam.