IPsec Tunnel is UP, But Traffic Still Doesn't Work?
  

George Fady Lv2Posted 2026-Jul-24 15:02

An IPsec VPN showing "Connected" doesn't always mean applications and users can communicate successfully.
I recently worked on a case where a Sangfor NSF established a successful IPsec VPN with a FortiGate firewall. The tunnel came up without any issues, yet traffic only flowed in one direction.
Everything appeared normal:

  • Tunnel status: UP

  • Phase 1 and Phase 2: Successful

  • Remote side could reach the Sangfor network

  • Sangfor could not reach the remote LAN

Rather than immediately modifying the VPN configuration, I followed a structured troubleshooting process:
• Verify the Security Policy (LAN → VPN)
• Check for Source NAT exemption
• Validate route selection using Route Testing
• Monitor IPsec encrypt/decrypt counters
Following these steps quickly narrowed down the problem without unnecessary configuration changes.
A healthy VPN tunnel confirms negotiation has succeeded—but it doesn't guarantee traffic is being processed correctly. Security policies, NAT, and routing still determine whether packets ever reach the tunnel.
In the next post, I'll share the actual root cause and how it was resolved.
Humayun Ahmed Lv4Posted 2026-Jul-24 17:27
  
Thanks to share!