I have a Vigor 2820n ADSL 2+ router.

I have disabled the PPTP, IPSec and L2TP VPN Services which should allow the PPTP VPN to pass through to the Windows 2008 SBS. I have also configured NAT to forward port 1723 to the Windows 2008 SBS where RRAS is running. Dont know how to forward GRE (Protocol 47).

I am still unable to connect a PPTP VPN to the server.

Can anyone suggest what I have done wrong?

Have you got any firewall rules set up to block incoming traffic? Other than that, there's nothing that would stop VPN passthrough from working unless maybe the port forwarding isn't working right, you do only need to port forward port 1723, GRE (UDP 47) is handled automatically.

Check which firmware you've got on the router, if it's 3.3.2.x then upgrade to 3.3.3 since that fixes issues with port forwarding.

I had the same problem,
We run an ISA 2006 server and use the 2820 as perimeter firewall
I solved it by
1) PPTP 1723 traffic - removing from port redirection and adding to open ports
2) Adding new filters to allow GRE 47 protocol in and out
3) Changing firewall filters to allow PPTP 1723 from any IP to any IP (VPN traffic was using multiple IP addresses to communicate so we had to open the scope to include all out WAN / LAN addresses)

It was very hard to find any info on the above senario and there was alot of trial and error using the syslog to work out what was going on.

I hope this post is useful to anyone else

I'm having the same problem, we switched out an old Draytek with a new 2920 and now VPN will not pass through to Windows. How do you Add a new filter to allow GRE 47 protocol in and out ?