Allow GRE packets in Cisco Router 877

Dear,
I have configured ISA Server 2006 as a VPN server and i tried within local network, it is working fine.

I tried from the external, the ISA server is accepting the connection but not receiving GRE packets and i get an error in the eventvwr saying ‘Router or firewall may block GRE packets’.
We have a Cisco 877 router with ADSL connection.I have already configured port forwarding for PPTP(1723) and GRE (47)protocol in the router and permit GRE packets but still it is blocking the GRE packets.

Can anyone please help me how to configure Cisco 877 to allow GRE packets for VPN client access.

Thanks and Regards

Abdul Rahuman.M