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Comcast Router blocking all ports with firewall disabled

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I have a new Cisco 5505 behind the Comcast cable modem/router (SMC).   I am using this device as a firewall and VPN router.   I have no problem making connections to the outside (using various protocols & ports).   I am unable to make any externally initiated connections into the internal network though (e.g. VPN via IPSec).  I do not have a static IP via Comcast.


I believe the firewall on the SMC router is disabled (I have checked the "Disable Firewall for True Static IP Subnet Only" - which Comcast tech support stated was the way to disable the firewall on this device (even though I don't have a static IP)).  I also have disabled "Gateway Smart Packet Detection".   Even though the firewall is supposedly disabled, I have also forwarded the necessary ports (500 & 4500) on the SMC.


When I attempt to connect to those ports, they are blocked.  When I perfrom an nmap port scan, all ports show as filtered (I can ping the device).  A capture on the 5505 external interface to the SMC internal interface shows no traffic during the nmap scan. 


I contacted Comcast tech support again, and explained the situation and testing I'd done, and they stated that the device is open and they are not blocking any ports.  I asked the support tech to test the SMC device's public IP and he stated he could not do that & that they don't block any ports, so the problem is with my 5505.  I'm at a loss.  Any ideas or things I've overlooked before I ask for a different modem/router?