Why Is IPsec Phase 1 Up but Phase 2 Down?
You run the CLI command 'diagnose vpn ike gateway list' and see that an IPsec VPN gateway is in 'up' state with 'initiator' mode, but no Phase 2 selectors are established. What is the most likely cause?
Quick Answer
The answer is mismatched Phase 2 selectors (proxy IDs). When an IPsec VPN shows Phase 1 up but Phase 2 down, the IKE SA has been successfully negotiated, meaning authentication and key exchange for the control channel are complete, but the IPsec SA—which carries actual traffic—cannot be established because the local and remote proxy IDs defining the traffic selectors do not align. On the Fortinet NSE 4 exam, this scenario tests your ability to interpret `diagnose vpn ike gateway list` output and distinguish between Phase 1 and Phase 2 failures; a common trap is assuming firewall policies are the root cause when Phase 1 is up, but the real issue is almost always selector mismatch. Remember the memory tip: “Phase 1 shakes hands, Phase 2 needs matching plans”—if the proxy IDs don’t mirror each other, the tunnel stays half-open.
⚠ Common exam trap
Candidates often confuse Phase 1 and Phase 2 failures, assuming an 'up' gateway means the entire VPN is working, but the 'initiator' mode and lack of Phase 2 selectors specifically point to a proxy ID mismatch rather than a proposal or connectivity issue.
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
✓
The local and remote Phase 2 selectors (proxy IDs) do not match
When an IPsec VPN gateway is in 'up' state with 'initiator' mode but no Phase 2 selectors are established, it indicates that IKE Phase 1 (main mode or aggressive mode) has completed successfully, but Phase 2 (quick mode) negotiation has failed. The most common cause for Phase 2 failure is a mismatch in the Phase 2 selectors (proxy IDs), which define the local and remote subnets to be encrypted. FortiGate uses these selectors to match traffic and establish the IPsec SA; if they do not align between peers, the Phase 2 negotiation will not complete.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The Phase 2 proposal parameters (encryption, authentication) do not match between peers
Why it's wrong here
While possible, the Phase 2 proposal mismatch typically causes 'no proposal chosen' and Phase 2 fails without establishing. The gateway shows 'up' so Phase 1 is fine.
- ✗
The remote gateway is not responding to IKE packets
Why it's wrong here
If the remote gateway was not responding, Phase 1 would not be up.
- ✓
The local and remote Phase 2 selectors (proxy IDs) do not match
Why this is correct
Mismatched proxy IDs prevent Phase 2 negotiation from completing successfully. The Phase 1 can be up but Phase 2 fails to establish.
- ✗
The IPsec interface is down
Why it's wrong here
An interface down would affect Phase 1 as well; the gateway would not be up.
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.
Go deeper
Related to this question
About these practice questions
This NSE4 question is part of Courseiva's 282-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 →
Same concept, more angles
1 more way this is tested on NSE4
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate is configured with an IPsec VPN to a remote site using IKEv1. The VPN tunnel goes down intermittently. The admin runs 'diagnose vpn ike gateway list' and sees 'state=UP' but no Phase2 selectors. What is the most likely cause?
medium- A.The firewall policy allowing IPsec traffic is misconfigured
- B.The remote gateway has a different PSK
- ✓ C.Mismatched Phase2 parameters between the local and remote gateways
- D.Dead Peer Detection (DPD) is disabled
Why C: When 'diagnose vpn ike gateway list' shows the IKE gateway state as UP but no Phase2 selectors are present, it indicates that the IKE Phase1 (main mode or aggressive mode) has completed successfully, but the IPsec Phase2 (quick mode) negotiation has failed. The most common cause for this is a mismatch in Phase2 parameters such as encryption algorithm, authentication algorithm, or proxy IDs (local/remote subnets) between the local and remote gateways. Since the tunnel goes down intermittently, the Phase2 rekey may be failing due to these mismatches, causing the tunnel to drop until a successful renegotiation occurs.
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.