In case anyone experiencing intermittent IPSEC VPN, and your machine is using HA. Please check your uptime. And if your uptime is more than 497 days.
You should check CSB-150807-1 (FortiGate HA failover after 497 days uptime).
Potentially Affected Products:
All FortiGate models running in HA cluster.
Potentially Affected OS:
FortiOS 4.0
FortiOS 5.0
FortiOS 5.2
The bulletin is not saying anything that this issue will impact to VPN. But in my case, seem this one impact to DPD.
We are using FGT3600 FortiOS 5.0.
In my case i got more than 20 tunnel that randomly down automatically. I can see phase1 and phase2 is up. But after several second, dpd will bring down the tunnel.
So, what i do is disable DPD on my tunnel. After that everything is OK. This is a temporary solution until we got a maintenance window to reboot the machine.
Salam//tato
