I have been trying for some time to get PPTP passthrough working. I know that I need port 1723 and GRE. I have 1723 working, but the GRE packets aren't getting through (using Windows VPN, it hangs on verifying user/pass). I am using CCP. I have port 1723 NAT'd to the internal PPTP server, and I have told the system to inspect GRE. I have done exactly this on several 871/851 routers and have never had any issue, but this new 861 seems to be giving me a lot of grief.
Has someone else got a simple fix for me?