Network Management Strategies for the CIO
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A Windows VPN connection will return error code 721 when the VPN server is not responding. The most common reason for this error code is a device that is blocking IP protocol 47 (GRE, used by PPTP) or IP protocol 50 (ESP, used by L2TP/IPsec).Look at the network path between your Windows XP client and your Win2000 server. Is there any device along this path, like a router or firewall, which might be blocking these protocols? If so, look for a "VPN pass through" configuration option on that device. Also consider any firewall software running on your Windows XP client.
Another possibility is that your Windows XP client is attempting L2TP/IPsec but your Win2000 server is configured for PPTP only. That's consistent with a Windows 98 client connecting, but a Windows XP client failing, since Windows 98 only supports PPTP (unless you added a Win98 L2TP/IPsec client). Look at your Windows XP VPN connection's Properties Panel Networking tab to see what type of VPN it is using. Try setting XP to request a PPTP VPN connection to see if that fixes your problem. If so, you've narrowed the problem down to L2TP/IPsec.
If you really wanted to use L2TP/IPsec, then look at both your XP client's and 2000 server's IPsec configurations to make sure that all security parameters match, including any preshared key used for authentication, encryption algorithm, integrity algorithm, and host/subnet addresses and masks. Also make sure that both protocol 50 (ESP) and port 1701 (L2TP) flow freely between your XP client and your Win2000 server. Further information on this topic can be found at Microsoft's website.
This was first published in July 2004
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