My ISP's cable modem is "exposing itself" between LAN workstations, and VPN destinations.
I have a FortiGate 61E at a remote site with a VPN tunnel back to my main site. I have had a problem where my Windows clients will detect a new network, and will identify it as "Network 3". That is, when you choose a type of Network in Windows 7, you choose from Home, Public, or Work, and then it gives the network a name. So it's almost as if the Windows machine thinks that the network has changed. This happens rather randomly throughout the day, several times a week, with different computers.
Yesterday I was doing some troubleshooting and I discovered that it's selectively unable to reach *certain* networks through the VPN tunnel. If I do a traceroute, it's picking up an address of 192.168.0.1 between the local gateway (the FortiGate) and the remote end of the VPN tunnel. The packets are dropping at 192.168.0.1. I do not use this IP address anywhere in my network! Oddly enough, I can browse to this IP address, and it loads a Ubee Modem DOCSIS page. I called my ISP, Spectrum (formerly Time Warner) and asked for an explanation. They said that everything is configured properly, but agreed that I should not be able to see that page or reach that address. In fact - I don't see how it's possible. The source address was 10.2.20.153 and the gateway (the FortiGate) is 10.2.20.1. How is it able to reach 192.168.0.1?
Perhaps there is something I am doing wrong in my configuration that is allowing that address to sneak into the routing path? Or do I need to contact the ISP and demand a different make/model of modem?