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What PPTP-VPN problems are caused by Windows XP SP2?

My customers have two Cisco routers connected to the Internet, which I configured to allow PPTP-VPN access from the customers' homes. These configurations worked well with Windows 2000 and Windows XP SP1 PCs, but since we upgraded the PCs to Windows XP SP2, none of them can establish PPTP-VPN connections to the routers. We have done many tests, including deactivating the PCs' firewalls, but the Windows XP SP2 PCs still fail to connect. What can I do to solve this problem?

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According to this Microsoft Knowledge Base entry, there are registry entries for PPTP in SP2 that you may need to set or update. Microsoft and others have noted that Windows XP SP2 introduced problems for VPNs, such as L2TP/IPsec, that require NAT traversal (NAT-T). While these problems shouldn't affect PPTP, PPTP is very insecure and you should consider upgrading to L2TP/IPsec if you have anything greater than casual security requirements. If you do decide to upgrade, see this Knowledge Base article for information on fixing the NAT-T problem.

Visit our Learning Guide for Windows VPN troubleshooting tips.

This was first published in October 2006

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