Whenever I try to connect behind a NAT to a VPN that uses
IPSEC (e.g., using a Cisco VPN client), it fails. If I try
connecting behind a NAT to a VPN using PPTP it works fine.
I followed the steps in KB 818043, but to no avail. The
only way I can get this to work is if I change my network
location to be outside the NAT.



Louise Bowman [MSFT]

Is this your scenario
Client----> NAT ----Internet---->Server

If the server is a Windows 2000 server this isn't going to work..

See the following:
Note If you apply the 818043 update to a Windows 2000-based server that is
using Routing and Remote Access, the server cannot function as an L2TP/IPSec
server in these scenarios. It cannot allow connections from L2TP/IPSec
clients when one or more NAT routers is involved. This update is a
client-side update only. Server-side NAT-T functionality is a new feature in
Windows Server 2003 Routing and Remote Access only. NAT-T server-side
support will not be added to Windows 2000 Routing and Remote Access

Louise Bowman
Microsoft Windows Networking Division

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