When you enable RRAS on your Windows 2000 server, TCP/IP forwarding is silently enabled on all interfaces.
TCP/IP forwarding is required for RRAS to forward client traffic on the LAN. Without TCP/IP forwarding, RAS clients would only be able to access the RRAS server, not the entire network.
If you wish to disable TCP/IP forwarding, use Administrative Tools / RRAS and right-click the server name. Press Properties / IP and clear the Enable IP Routing box.