Strange Network Problem

While I do consider myself relatively versed in FreePBX, when it comes to networks, I’m not very bright. However, I’m experiencing a problem that seems a little strange. Although I’m pretty sure it’s a network problem, I was wondering if the specifics could narrow down it’s cause.

Basically, we have a remote server and another location with a number of extensions. Those extensions can register, at least temporarily, and make calls between each other and out. However, in a few minutes, soon weird stuff starts to happen. Dialing another number may call the incorrect extension, or even itself. I can always call external numbers, and the echo test seems to also work, it’s just when I call other “internal” extensions that I encounter this problem.

I’m pretty sure we’re double natted, which could be causing problems. Although our isp says our modem is in bridge mode, I can still access the modem with a local ip, so I think they’re wrong. I turned off sip alg in both the router and modem, but am still experiencing this problem. Also, from a totally different network, things seemed to work ok, which points even more to a network problem. If it makes any difference the modem is an SBG6580 and the router is a Cisco RV325

A clue is that the wrong extension rings, that suggests that your nat traversal is incomplete, if you have ‘n’ extensions behing a nat box, then you need all your firewall/routers to provide a set of ‘n’ connections on DIFFERENT ports that remains consistent and maps the incoming/outgoing connctions from/to that "n’th"extention through probably both natted routers to the VOIP server. (that is 4 places that stuff happens) so debug your SIP traffic from the Asterisk box, at both routers , and at the phone itself if possible. At some point in that change you will see a misdirection in the various SIP headers that are the basic cause your problem.

Alright. I ran out of time to decipher those problems. I ended up setting them up on a vpn through the system admin. That seemed to fix our problems. Is using a vpn a common solution to network problems? Why would doing this prevent network problems? It seems like nat issues would still cause problems even when using a vpn.

It’s certainly a common solution for this type of double-natted network.

When using a VPN, you are changing the “nat at both ends” to “routing at both ends”. You are traversing the intermediate network on a routable connection, where before, you were setting up temporary tunnels that weren’t consistent at both ends, you are now setting up connections that are consistent at both ends.

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