If you have a RAS client that disconnents "immediately" (during or shortly after authentication), you may have a duplicate IP address.
You may also find an event on the RAS server similar to:
Event ID: 20050 Source: RemoteAccess Description: The user DOM\User connected on port Com1 onThis can be caused by:
A client is using an IP address that is in the RAS servers' RAS IP static pool.
RAS IP static pool addresses exist on the local LAN.
DHCP is used for the RAS address pool and the DHCP server has a rebuilt database (and you haven't renewed the lease).
To resolve the problem:
Disconnect all RAS clients.
Ping every IP address in the RAS pool. If you get any response, change the RAS server pool address or change the LAN workstation address.