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NSE4 · Question #288

A FortiClient fails to establish a VPN tunnel with a FortiGate unit. The following information is displayed in the FortiGate unit logs: msg="Initiator: sent 192.168.11.101 main mode message #1 (OK)"…

The correct answer is A. An IPSec DHCP server is not enabled on the external interface of the FortiGate unit. The VPN tunnel failure, indicated by 'Failed to acquire an IP address' in the logs, is likely caused by the FortiGate not having an IPSec DHCP server enabled to assign IP addresses to connecting FortiClient users.

Submitted by dimitri_ru· Apr 18, 2026VPN and Routing

Question

A FortiClient fails to establish a VPN tunnel with a FortiGate unit. The following information is displayed in the FortiGate unit logs:

msg="Initiator: sent 192.168.11.101 main mode message #1 (OK)" msg="Initiator: sent 192.168.11.101 main mode message #2 (OK)" msg="Initiator: sent 192.168.11.101 main mode message #3 (OK)" msg="Initiator: parsed 192.168.11.101 main mode message #3 (DONE)" msg="Initiator: sent 192.168.11.101 quick mode message #1 (OK)" msg="Initiator: tunnel 192.168.1.1/192.168.11.101 install ipsec sa" msg="Initiator: sent 192.168.11.101 quick mode message #2 (DONE)" msg="Initiator: tunnel 192.168.11.101, transform=ESP_3DES, HMAC_MD5" msg="Failed to acquire an IP address Which of the following statements is a possible cause for the failure to establish the VPN tunnel?

Options

  • AAn IPSec DHCP server is not enabled on the external interface of the FortiGate unit.
  • BThere is no IPSec firewall policy configured for the policy-based VPN.
  • CThere is a mismatch between the FortiGate unit and the FortiClient IP addresses in the phase 2
  • DThe phase 1 configuration on the FortiGate unit uses Aggressive mode while FortiClient uses

How the community answered

(49 responses)
  • A
    86% (42)
  • B
    4% (2)
  • C
    2% (1)
  • D
    8% (4)

Why each option

The VPN tunnel failure, indicated by 'Failed to acquire an IP address' in the logs, is likely caused by the FortiGate not having an IPSec DHCP server enabled to assign IP addresses to connecting FortiClient users.

AAn IPSec DHCP server is not enabled on the external interface of the FortiGate unit.Correct

The log message 'Failed to acquire an IP address' directly indicates that the FortiClient could not obtain an IP address once the VPN tunnel negotiation was complete. For FortiClient IPSec VPNs, the FortiGate unit typically acts as a DHCP server to assign IP addresses to connecting clients; if this server is not enabled or properly configured on the interface handling VPN connections, clients will fail to get an IP.

BThere is no IPSec firewall policy configured for the policy-based VPN.

The logs show successful Phase 2 quick mode messages, which implies that a corresponding IPSec firewall policy is configured and allows tunnel establishment, ruling out its absence as the cause of an IP acquisition failure.

CThere is a mismatch between the FortiGate unit and the FortiClient IP addresses in the phase 2

A mismatch in Phase 2 selectors (IP addresses/subnets) would cause Phase 2 negotiation itself to fail, not an IP address acquisition failure *after* Phase 2 reports 'DONE'.

DThe phase 1 configuration on the FortiGate unit uses Aggressive mode while FortiClient uses

Phase 1 mode mismatch would cause Phase 1 negotiation to fail, but the logs show `main mode message #3 (DONE)`, indicating successful Phase 1 establishment.

Concept tested: FortiGate IPSec VPN Client IP Assignment

Source: https://docs.fortinet.com/document/fortigate/7.4.0/administration-guide/526779/ipsec-vpn

Topics

#VPN Troubleshooting#IPsec VPN#Remote Access VPN#DHCP for VPN

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