Article ID: 888201 - View products that this article applies to.
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:
Note The 721 error description may vary.
Error 721: The remote computer is not responding.
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:
Event ID: 20073
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:
241251For more information about VPN security, click the following article number to view the article in the Microsoft Knowledge Base:
(https://support.microsoft.com/kb/241251/ )VPN tunnels - GRE protocol 47 packet description and use
255784For 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:
(https://support.microsoft.com/kb/255784/ )Increasing security on Windows 2000 VPN server
323441For more information about the PPTP protocol, visit the following Microsoft Web site:
(https://support.microsoft.com/kb/323441/ )How to install and configure a Virtual Private Network server in Windows Server 2003
Article ID: 888201 - Last Review: December 4, 2007 - Revision: 1.6