You receive an "Error 721" error message when you try to establish a VPN connection through your Windows Server based remote access server

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.

Cause:

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.

Resolution:

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.

Reference:

• http://support.microsoft.com/kb/888201
• http://support.microsoft.com/kb/241251