I agree this looks a little bit convoluted.
Basically, there is only ONE configuration on a HA cluster (apart from the hostnames and some device specific HA settings). So if your cluster fails over the second WAN line configuration has to be (more or less) identical to your first one, dial-out parameters, IP addresses, gateway etc.
And with each FGT failover you would switch WAN lines...who would do that without being forced to? Of course you' ll lose all sessions and spend a good time waiting for connectivity to be re-established. There must have been a lot of good reasons for this design, or maybe not enough time to think it over.
You can have 1 VPN tunnel interface on WAN1 and have the cluster maintain it even when failing over. Of course, the slave unit needs to be connected to WAN1 as well. That' s device redundancy. What your setup is trying is to achieve WAN (ISP) redundancy as well with the same setup.
IMHO you could set this up this way:
- configure 2 default gateways to ISP1 and ISP2, resp., using different distances to prefer ISP1; will switch over automatically to ISP2 if ISP1 is determined to be dead (enable dead gateway detection/ping server on the WAN interfaces)
- to get your devices to connect to the second WAN IP in case of failover you could set up a DDNS entry and terminate the VPN tunnel on that dynamic hostname instead of a fixed IP. I assume the (external) devices dial in, and the tunnel doesn' t need to be up for internal->external traffic.
In your current design, your devices generate a lot of traffic that is never answered, i.e. try to build up the second tunnel endlessly, and send traffic down a non-connected tunnel etc. This is crude.
One last good advice: if you' re from a different IT field then I' d strongly recommend to get professional help from a FGT partner onsite. Seems like you' re willing to make efforts to get the incoming data lines set up redundant so maybe your company could spend some more money at the right spot, namely on design, setup and testing. This isn' t exactly what you set up every day in 10 minutes.
Ede Kernel panic: Aiee, killing interrupt handler!