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New Member

Voice Chat using MSN Messenger behind NAT


I recently installed a Cisco 1721 Router that is doing NAT for some Host PCs. These PC's are configured with private IP Addressing.

Problem is with MSN Messenger Voice. Since MSN Messenger uses SIP for establshing connection, and i think it embeds the IP Address information in Layer 5\6, so it is not working.

Can someone point me towards a configuration for Cisco IOS, that would be able to acheive this ? I.e. ability to use SIP , while using NAT (PAT or Static) ?

I am running 12.2(11)T.

Regrds \\ Naman

  • Video Over IP

Re: Voice Chat using MSN Messenger behind NAT

Try the following command, need to specify the tcp and/or udp port # as well.

ip nat service sip ...

New Member

Re: Voice Chat using MSN Messenger behind NAT

I did try with this command, however didn't work. I might be wrong about the port number, I tried 1900 ?

Does anyone know what it could be ?

I captured packets for a few sessions and it was trying to establish SDP session on port 1900, that why i used that number.

New Member

Re: Voice Chat using MSN Messenger behind NAT

The default SIP port is 5060, however as SIP uses many different ports for communication allowing just 5060 will not work.

Cisco PIX Firewall versions 6.0 and 6.1 offer SIP support that solves NAT traversal issues by acting as an application-layer gateway (ALG).

Cisco PIX Firewalls dynamically open and close UDP ports for secure SIP traffic to flow through. Other vendors must open a large range of UDP ports, which creates a security risk

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