SIP and Port Forwarding

[StatonDaFacts]

I am working on a project for my Work on my home network I have 2 Routers behind my mmy "ZHONE" Modem provided by my ISP My Main Router Is a ASUS RT-AC88U MERLIN Used so the Kids can have uninterrupted Network access so they can do their homework and I can filter out PORN... and on my second router NETGEAR 7500 running DDWRT running STRONG VPN
I am unable to get port forwarding to my SIP GRANDSTREAM PBX UCM 6108 If I put it on the first router it works fine. second nothing. I cant even get any ports to forward to my second router with or with out strong vpn running.. Ive port forwarded from router 1 192.168.1.1 to router 2 192.168.20.1 even from router 1 192.168.1.1 to router 2 WAN 192.168.1.79 Also tried DMZ and port triggering. I have been at this 3 nights a week for the past month and the Wife Is getting Annoyed that I spend so much time infront of my PC.... ANY IDEAS...... Also my end goal is to have the SIP PBX box on the strong VPN

Mod Edited Title

 

[nigelivey]

Research "double NAT"

 

[bill001g]

In many cases you will have to put a SIP device in DMZ mode. With SIP is there is a control stream that is TCP. That is pretty straight forward and can make it though nat or has a common port you can forward. The problem comes when you actually make a phone call. Inside the tcp data connection the 2 devices negotiate what ports the actually voice traffic is going to use. Then then both independently open sessions to the corresponding UDP port. There is not way to predict these ports so you will just suddenly get a incoming UDP session on what appears to be a random port.

The only solution for this is the router needs a special feature that can detect this SIP communication and spy on it and extract the negotiation of the ports so it is dynamically allowed. Some router have this and some do not.

Now this feature will not work if you run SIP encrypted which you really should be doing if you are running it over the internet. This leaves DMZ the only method that then works.