Asterisk/Freepbx problem with comcast & double NAT

I switched my asterisk system from a static IP to a Comcast Business cable modem with dynamic ip. I thought my system was running ok (I could dial out etc.), but then someone told me they couldn’t dial in.

After a lot of futzing about, I found that asterisk/freepbx doesn’t work well with with double NAT (i.e. cable modem with NAT + tomato router), but in a non-obvious way. You can dial in to the IVR, but all of the internal phones are always busy.

It’s not easy to get the cable modem to bridge. Comcast wants $14.95/mo for static IP, which I find outrageous and refuse to pay. I tried (a) turning off the firewall in the Comcast modem (SMCD3G-CCR) and (b) put my tomato router in the DMZ of the cable modem. Neither helped.

So, for now, I’ve switched back to Does anyone have any suggestions as to how to get asterisk to work with Comcast? (or a least a more detailed explanation of why DMZ and/or no firewall doesn’t help…)

There is some hope that the Motorola SB6121 cable modem allows bridge mode - I’m going to try that next.

The SB6121 modem works in bridge mode. I have the 6580 version on TWC with the wireless turned off and in bridge mode with DDWRT on a Buffalo N router at my house.

I work at home quite often, run a IPSEC VPN appliance behind the firewall back to my office and management network for the data centers. I am always plugging and unplugging voip stuff to test and occasionally I will have to flush the ARP table on the router.

Go for Bridge mode and enjoy your DOCSIS 3 speeds.

If you need any help on bridge mode ping, I just tried to get into the Motorola and realized I misplaced the password and don’t know the management IP but I can hunt it down if I need to.