I have Peplink Balance 20 with 7.0.1 build 3414 and I have bad troubles with L2TP/IPSec passthrough. I have one CISCO and one Huawei 4G router connected to Balance. There’s one Windows Server 2016 in LAN that works as VPN server. CISCO has IPSec passthrough enabled and it forwards all external traffic to Balance. SSTP based VPN connections from external network work fine.
The problem is with L2TP/IPSec connections. When connecting to Windows Server 2016 from LAN using L2TP/IPSec, everything works fine. Connections are established and they are stable. But I have had no luck getting these connections work from external network.
I tried to play with IPSec and related NAT settings. I tried IPSec passthrough. I also tried port forwarding and NAT mappings. All with no luck. All I get back is error “789: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.”
What is the correct setup for Balance to make L2TP/IPSec passthrough so the Balance router just routes all this traffic to my Windows Server 2016 and doesn’t involve itself to this drama?