Cisco Router not disconnecting.

We have installed Windows Nt 2000 Server.
One NIC serves the LAN and the other NIC is connected to a Cisco 800 router. Without DNS installed the router is working fine. After 3 minutes of inactivity it disconnects from the line. After installing DNS the router does not disconnect. It seems there are messages sent by the DNS server to the router and this resets the disconnect timer.
Where can I find the solution for this problem?
SP1 is installed.
Hope someone can help us.