How do you add a GRE 47 exception to a VPN PPTP Windows 2008 Server?

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http://serverfault.com – I’m trying to set up a PPTP VPN connection on a Windows 2008 R2 SP1 Server with a public IP. One of the steps is to allow GRE (Generic Routing Encapsulation) 47 direct access to the server. How can I make an exception in Windows Firewall, when it isn’t an actual port, but a protocol? (HowTos)