IPsec only works when MTU of WAN is set to 1500 !?!?
I have got a static internet connection with no PPPOE, PPTP, L2TP or anything else on both sides of an IPsec VPN connection. Just plain old IP connection with static IPs and MTU 1500.
Both sides have ALIX boards with pfSense 1.2.2 installed.
IPsec connection is established and I can ping from one site to the other and vice versa.
But RDP remote desktop connection for example only works when I set the MTU of both WAN interfaces to 1500. It doesn't work if I leave the MTU box empty.
It costs me lot of time to solve this problem and I hope I can help some others with the same problem.
If someone knows the reason for this behaviour I am very interested!