Hello,
I need some help to configuration of my ROS machine.
Please check diagram belows. I have VPN server at IDC with Public IP Address 234.123.234.2 (from ISP3)
and oversea VPN Client can access directly to this server.
Problem is My IDC's oversea bandwidth is almost full and they can't increase oversea bandwidth to my VPN server.
Fortunatley, Our Inter-ISP-Exchange Network is very powerful enough.(at least 10Gbps assigend for each ISP.) So i Leased one more internet connection from another ISP (ISP2) which have enough oversea bandwidth and connect to my ROS Machine. (IP 123.234.123.2)

What i want to do...

1. Oversea Client calling to 123.234.123.2 with VPN Client software,
2. My ROS Machine forward all traffic to VPN Server
3. Downstram for client from server also passthrow My ROS Machine again.

If it possible,

I want to lease few more internet connection from differnt ISP for differnt location. then traffic load balancing to use multi-location ROS Machines.
If then customer can select each ISP's Public IP Address for oversea VPN traffic as a fake VPN server.
Port forwarding tcp1723&gre is only effect to upstream.i need downstream too.
Anyone have a idea?

P.S. Maybe i can put VPN server to each location. but it mean also need more powerful machine required for each location. My main VPN server is powerful enough. only need oversea bandwidth. that's all.

The problem is that the default route back from the client won't be through the new ISP #2 like you said.
The only solution that I know of is that you let your client connect to the public IP given by ISP #2 and then you forward that PPTP connection to your VPN Server.

I hope for your sake that you've given your clients a hostname and not a static IP address. This way you could let the DNS servers give the public IP address at ISP #2 to the overseas clients.

I'm not an expert in this network related stuff, so you should get atleast a second opinion.