NSE4 Authentication and VPN Practice Question
A site-to-site IPsec VPN is configured with IKEv2. The tunnel establishes but traffic does not pass. Which two troubleshooting steps should the administrator perform first?
⚠ Common exam trap
Test-takers frequently assume a 'tunnel up' status guarantees traffic flow, but FortiGate separates tunnel negotiation from firewall policy enforcement, so both a policy and a route are required for traffic to pass.
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
✓
Check the firewall policies allowing traffic through the tunnel.
Even if the IPsec tunnel is established, traffic will not pass unless firewall policies explicitly permit it. In FortiGate, a Phase 2 tunnel being up does not imply that traffic is allowed; you must have a policy that matches the source/destination and enables the action to forward traffic through the tunnel interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the Phase 2 selectors.
Why it's wrong here
Phase 2 is already up, meaning the traffic selectors have been successfully negotiated between both peers. Re-checking the selectors cannot resolve a data-plane connectivity issue because the SAs are healthy and capable of carrying traffic. The real problem is likely outside the IKE/ESP negotiation, so focusing on selectors is both misleading and time-consuming.
- ✗
Verify that the Phase 1 proposal matches.
Why it's wrong here
The Phase 1 proposal being already up confirms that the IKEv2 parameters—such as encryption algorithm, integrity algorithm, DH group, and PRF—matched during the initial handshake. Since the IKE SA is established, verifying the proposal again is futile; no parameter mismatch persists. The failure to pass traffic must be attributed to another layer, such as forwarding or policy enforcement.
- ✓
Check the firewall policies allowing traffic through the tunnel.
Why this is correct
In Fortinet's policy-based VPN model, firewall policies are the gatekeepers that decide which traffic is allowed to traverse the tunnel interface. Even with Phase 1 and Phase 2 both up, packets are dropped unless there is an explicit policy permitting traffic between the local and remote zones (e.g., from 'internal' to 'tunnel'). This is a frequent point of failure because the tunnel status itself is independent of the policy configuration.
- ✓
Check the routing table for routes pointing to the remote networks.
Why this is correct
The routing table determines whether a packet destined for the remote network is sent to the IPsec tunnel interface at all. Without a static or dynamic route that points the remote subnet to the tunnel, the FortiGate will forward the packet via the default route or drop it, bypassing the tunnel entirely. This is distinct from a firewall policy issue; even with perfect policies, missing routes prevent the tunnel from being used.
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
About these practice questions
This NSE4 question is part of Courseiva's 773-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
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.