Error 721: Remote PPP Peer or Computer is not Responding.

Error 721 results when you try to establish a VPN connection through your Windows device to a corporate network by using a Point-to-Point Protocol (PPTP) client. The connection to the Microsoft Windows Server-based remote access server may not always succeed.

Typically, Error 721 occurs when your ISP/Network does not allow PPTP Port 1723 or GRE packets. To rectify error 721, please check the router settings and make sure TCP Port 1723, IP Protocol 47 (GRE) are allowed. Also, make sure that the router has the PPTP and PPTP pass-through option enabled.

Furthermore, you may switch between L2TP, SSTP, and OpenVPN protocols.

Follow this link if you’re using Windows XP: http://support.microsoft.com/kb/810839

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