A few months ago we had a problem with a dialup VPN and RDP connections to our DC from our remote offices. Our ISP installed ZTE and ISKRATel modems in those offices.
Employees complained to lose connection with the server when using RDP and Outlook.
By monitoring VPN sessions on the ISA server I couldn’t noticed any network connectivity problem and dynamic IPs remained same, but I did noticed that some remote offices get different IPs (always starting with 92.x.x.x or 109.x.x.x).
After 2-3 months the problem has been solved and from the ISP’s told me that problem is due to dual-stack and offices now get only IPv4 addresses (92.x.x.x).
At the headquarters we have a Cisco 1841 router and I’ts configured to port forward well known ports (rdp 3389 and 1723 and gre 47).
The question is where roughly could be the core problem: ISP? My router? my DC server? DSL modems? Anything else?