Troubleshooting IPSec VPN Issues on FG-90G Firmware 7.4.11

11, covering phase negotiation, routing, policies, and packet-flow validation.

Quick Read

Symptom: 11, covering phase negotiation, routing, policies, and packet-flow validation.
Check first: Capture the affected source, destination, protocol, port, DNS name, VLAN or subnet, and exact error before changing policy.
Risk: Review before running

Symptoms

IPSec VPN is not establishing a connection on FortiGate 90G running firmware version 7.4.11.

Environment

FortiGate 90G firewall, running firmware version 7.4.11, configured for IPSec VPN.

Most Likely Causes

Potential misconfiguration in VPN settings, incorrect phase 1 or phase 2 parameters, or network connectivity issues.

What to Check First

Capture the affected source, destination, protocol, port, DNS name, VLAN or subnet, and exact error before changing policy.
Verify path, name resolution, authentication, and firewall policy separately so one symptom does not hide multiple failures.
Check whether the issue is isolated to one client, one subnet, one VPN profile, or every path.

Fix Steps

Verify VPN Configuration
Check the IPSec VPN configuration for both phase 1 and phase 2 settings.
Example pattern only. Adjust for your environment before running.
```
config vpn ipsec phase1-interface
show
config vpn ipsec phase2-interface
show
```
Check Firewall Policies
Ensure that the firewall policies allow IPSec traffic through the FortiGate.
Example pattern only. Adjust for your environment before running.
```
config firewall policy
show
```
Inspect Logs for Errors
Review the logs for any error messages related to IPSec VPN connections.
Example pattern only. Adjust for your environment before running.
```
get log vpn
get log event
```
Test Connectivity to Remote Gateway
Ping the remote VPN gateway to confirm network connectivity.
Example pattern only. Adjust for your environment before running.
```
execute ping <remote_gateway_ip>
```
Check NAT Settings
Ensure that NAT settings are correctly configured for the VPN traffic.
Example pattern only. Adjust for your environment before running.
```
config firewall policy
show
config system settings
show
```
Verify IPsec SA Status
Check the status of the Security Associations (SAs) for the VPN.
Example pattern only. Adjust for your environment before running.
```
get vpn ipsec tunnel summary
get vpn ipsec tunnel details <tunnel_name>
```
Review Phase 1 and Phase 2 Parameters
Ensure that the encryption and authentication settings match on both ends of the VPN.
Example pattern only. Adjust for your environment before running.
```
config vpn ipsec phase1-interface
edit <tunnel_name>
show
config vpn ipsec phase2-interface
edit <tunnel_name>
show
```

Validation

The same client and network path can reach the target after the change.
Firewall, VPN, DHCP, DNS, or switch logs show allowed traffic or successful negotiation instead of the prior failure.
A second path check confirms that the fix did not open unintended access or break another subnet.

Logs to Check

Firewall, VPN, DNS, DHCP, or switch logs for the failing timestamp.
Client resolver, route table, VPN client, or browser/network diagnostics.
Packet capture or flow logs when policy and routing disagree.

Rollback and Escalation

Export or screenshot the original policy, route, resolver, or interface configuration before changing it.
Remove temporary allow rules, test DNS records, or route changes after validation.
Restore the previous VPN profile, firewall rule, or switch configuration if reachability worsens.

Escalate When

Escalate if the same error persists after rollback and a clean retry from the original failing path.
Escalate if logs show authorization, data loss, certificate, replication, or production availability risk outside the local service owner scope.

Edge Cases

Check for overlapping IP addresses between local and remote networks.
Ensure that the MTU settings are correctly configured to avoid fragmentation issues.

Notes from the Field

Most network incidents need source and destination evidence. A successful test from an admin laptop does not prove the affected client path is fixed.
For VPN and firewall changes, keep the blast radius narrow and time-box any temporary allow rule.