You receive an "Error 721" error message when you try to establish a VPN connection through your Windows Server-based remote access server
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If you try to establish a virtual private network (VPN) connection to a corporate network by using a Point-to-Point Tunneling Protocol (PPTP) client, the connection to the Microsoft Windows Server-based remote access server may not succeed. You may receive the following error message:
Error 721: The remote computer is not responding.
Note The 721 error description may vary.
If you are using a Microsoft Windows NT Server 4.0-based remote access server, the following event may be logged in the System log of the server:
This issue may occur if the network firewall does not permit Generic Routing Encapsulation (GRE) protocol traffic. GRE is IP Protocol 47. PPTP uses GRE for tunneled data.
To resolve this issue, configure the network firewall to permit GRE protocol 47. Also, make sure that the network firewall permits TCP traffic on port 1723. Both of these conditions must be met to establish VPN connectivity by using PPTP.
For more information about the GRE protocol, click the following article number to view the article in the Microsoft Knowledge Base:
241251 VPN tunnels - GRE protocol 47 packet description and use
For more information about VPN security, click the following article number to view the article in the Microsoft Knowledge Base:
255784 Increasing security on Windows 2000 VPN server
For more information about installing and configuring a VPN Server in Windows Server 2003, click the following article number to view the article in the Microsoft Knowledge Base:
323441 How to install and configure a Virtual Private Network server in Windows Server 2003
For more information about the PPTP protocol, visit the following Microsoft Web site: