VPN Tunnel problem

3 replies
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Hello,

We are experiencing the following issue in our company: we have a FortiGate 60F with a GRE over IPsec tunnel configured between a Cisco router and the FortiGate firewall.

When we checked PRTG, we noticed that the tunnel went down on February 27. Upon investigation, we found that both phases are configured identically, and the configurations are the same on both ends. However, the Interconnect does not establish a connection, and ping does not work.

Could you please help us understand what might be causing this issue? Ping is enabled on the interface, and everything was working correctly before. We have not made any changes on the FortiGate, and as I was informed, there were no changes on the Cisco router either.

Additionally, we switched the tunnel from Tunnel Mode to Transport Mode, but the issue persists.

I would appreciate any recommendations or troubleshooting steps. Thank you very much for your feedback!
FortiGate #VPN #Ipsec

Dhruvin_patel

Staff

Hello!

Please check this document that can help you to troubleshoot the IPSec tunnel connection.

https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-Troubleshooting-traffic-not-flowing-through/ta-p/198199

https://community.fortinet.com/t5/FortiGate/Technical-Tip-IPSec-VPN-diagnostics-Deep-analysis/ta-p/193811

Regards!

francesimaoGmail

New Member

After you check the links and troubleshooting steps provided in the previous reply, one likely cause of the issue could be Dead Peer Detection (DPD) or keepalive issues. If the devices lose connectivity even briefly, DPD might mark the peer as dead, causing the tunnel to drop. Another possibility is that the Internet Service Provider (ISP) may have started blocking or rate-limiting IPsec traffic, especially UDP ports 500 and 4500, which are crucial for IKE negotiations.

It's also worth considering if there was a firmware update on either device that modified default behaviors or security settings. Additionally, an expired IKE/IPsec security association (SA) or certificate (if used) could prevent re-establishing the tunnel. Lastly, hardware issues like overheating or resource exhaustion (CPU or memory) on the FortiGate or Cisco device could be interfering with tunnel stability.

Starting with checking the DPD status, reviewing logs for errors, and ensuring the ISP isn’t blocking necessary traffic would be good initial steps.