Comcast Bussiness Class and GRE Tunnels

[Nate Burke <nate () blastcomm com>]

Hello, I'm hoping that someone here might have run into a similar issue 
and might be able to offer me some pointers.

I have a customer that I am providing redundant paths to, one link over 
a microwave connection, and a backup link over a Comcast Business Class 
Connection.  Everything on the Microwave link is working fine.  On the 
Comcast Connection, I have a Static IP from Comcast, and I want to setup 
a vendor specific GRE tunnel (Mikrotik EoIP) from my NOC to the Comcast 
Static IP Address.  It looks like the SPI Firewall inside the SMC 
Gateway required by comcast is blocking the GRE packets, I'm basing this 
on the fact that when I power cycle the modem, I get 1 ICMP Packet 
through the GRE Tunnel while the modem is booting up, then it stops 
again.  I have gotten to Tier2 support who swears that all Firewalls on 
the SMC Gateway are disabled.

As a workaround, I was able to establish a PPTP tunnel to my NOC, 
however it seems like the tunnel will only run for a few hours, then 
becomes slow to the point of being unusable.  In my mind this would be 
no different than setting up a permanent VPN back to a corporate office, 
which I would think happens all the time, so I'm not sure why I'm 
running into issues with it.

Anyone with Insights or comments would be appreciated.

Thanks,
Nate Burke