how to set the network interface to use for openvpn client

[junkixl]

Hello,

I hope I have choosen the right forum for my question. Sorry, if I got something wrong.

NETWORK CONFIGURATION:
I have a mobile computer (Windows 7 Pro) which uses two network interfaces (Ethernet and Mobile Broadband (3G Modem)). The ethernet adapter is wired to a router which sets the ip/subnet/gateway with DHCP. This router, however, is NOT connected to the Internet. The mobile broadband adapter connects to the ISP and a special service which allows mobile devices to be accessed from the internet (mobile 2 mobile communication. Provider is mdex, if somebody cares). To be able to communicate with other mobile devices, I have to establish an openVPN connection with an mdex-server.

PROBLEM DESCRIPTION:
Everything works fine, as long as the ethernet cable is disconnected. OpenVPN uses the mobile broadband interface and connects to the server.
When an ethernet connection exists, OpenVPN can't connect anymore.

WHAT I TRIED:
I configured the ethernet with static IP and Netmask and WITHOUT any gateway. With this configuration OpenVPN is also able to connect to the server over mobile broadband.
With DHCP enabled, I tried to change the metric of the interfaces - without any luck. I also tried to remove the gateway with netsh but that only works with static ip's.

- Is there some kind of configuration switch to select a specific network adapter to use in OpenVPN?
- I've read a lot of posts talking about adding routes for the vpn connection. But since I can't even connect, that doesn't apply to my problem, right?

I'm kind of new to VPN and mobile connections. It would be awesome, if somebody could point me in the right direction.

Thanks in advance