Zscaler GRE tunnel goes down after sometime

I didn't use much GRE but the few times I used it I noticed connectivity issue. I recommend to use IPsec tunnel instead.

Hi,

Have you checked system event logs? If you have a GRE keepalives/monitor and if it is failing intermittently then it can cause issues with the GRE tunnel. Usually system log will be something which you can look at first.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-GRE-Tunnel-monitoring/ta-p/197720

Regards,

Shiva

Hello,

First step which you can do is to do a 'diagnose sniffer packet ' for the remote IP address of the GRE tunnel  when does not work to see if your device sends and receives packets from remote GRE tunnel.
Second is to check the link-monitor status if it's configured. If you have link-monitor and your FG does an icmp requests , check for DDoS protection on remote end .

Best regards,

Fortinet

Can refer to:

https://community.fortinet.com/t5/Support-Forum/GRE-Tunel-not-working-after-7-4-1-gt-7-4-3/td-p/298665/page/3

https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-GRE-traffic-stops-working-after-a-firmware/ta-p/308279