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New Member

Cisco 837 & SIP / VOIP

I have a Cisco 837 DSL modem/router, which includes an integrated 4 port switch. On the switch, port 1 is connected to a Linksys WRT54G router, which has my LAN connected to it, along with 13 VOIP phones which use SIP. Port 2 is connected to a Linux machine running Asterisk. The Linksys and the Linux box both have public IP addresses. The LAN is running on 192.168.1.X.

While I am able to ping from the LAN to the Linux machine, and SIP registration requests are being received by Asterisk, Asterisk is unable to get replies back to the IP phones. Generally, in other setups, this is caused by improper NAT translation.

I am not familiar with the 837 where I could diagnose or resolve this issue. If I put the Linux box on the Linksys and assign it a private IP, everything is fine. This leads me to believe the problem is inside the 837 somewhere - perhaps in the NAT/NAPT configuration.

Does anyone have any insight into this? Any assistance would be appreciated.

Super Bronze

Re: Cisco 837 & SIP / VOIP


I think what you have is the 192.168.1.x LAN behind the Linksys.

The Linksys is then doing PAT to it's 'external' IP address which is connected to the 837s 4-port switch.

Phones can reach the Asterisk, however whilst the IP addresses on the packets are being NATted, the SIP packets probably have the '192.168.1.x' addresses embedded in them which are not accessible to the Asterisk.

When you move the Asterisk, you move it behing the Linksys NAT so it works fine.

What NAT do you normally configure that works? Have you a similar setup elsewhere using Linksys kit?


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Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
New Member

Re: Cisco 837 & SIP / VOIP


Thanks for the reply.

Almost all of our other clients have devices behind Linksys NAT, registering to SIP servers either on our network or through a device other than a Cisco 837. The Cisco 837 is the only new/difference piece of equipment in this configuration, unfortunately.