I have a Thomson TG585v8 router and I connect to a Cisco VPN using my Macbook running Mavericks. When I connect to the VPN all other devices in the house lose connectivity to the internet! I checked the router logs and I got quite a few errors like the following when connecting to the VPN:

UPnP action ‘AddPortMapping’ from ip (Undocumented UPNP error)

It is clearly and error related to the uPnP. So I disabled UPnp on the Thomson router and now everything works perfectly when I connect to the VPN.