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Community Member

Cisco 2600 Sip-UA over a NAT?


I'm trying to configure a 2600 with fxs cards as a sip-ua to communicate with an ISP's sip network. I've run a couple of debugs and my sip packets are being sent out as my LAN ip range which I suspect could be the reason i'm not receiving any reply messages. I've tried creating a loopback on that router with the inside global ip address with no success. I'm wondering if there are any options? I've seen information on session border controllers which can modify the SIP headers for me but I'd need to upgrade routers. I've also seen that on ATA186/188 you can specify the WAN IP address but that means buying an ATA.

Any help would be great!



Hall of Fame Super Gold

Re: Cisco 2600 Sip-UA over a NAT?


What IOS are you using ? Is the NAT being done by the router or other device ?

The router should source SIP requests with the interface address that has route to the provider.

Community Member

Re: Cisco 2600 Sip-UA over a NAT?

The ios is 12.3(8)t with ipvoice.

The nat is being performed by another device.

Hall of Fame Super Gold

Re: Cisco 2600 Sip-UA over a NAT?

Can you confirm that SIP messages are sent the with the wrong source address, by applying an access list on the outside interface ? Either if you deny or permit traffic sourced from the wrong interface, you will have a counter about this.

Also, can you confirm messages that messages have the correct from address in the SIP message ?

Also I would suggest you upgrade at least to latest maintenance of 12.3T. That should fit even in the limited memory of the 2600.

Community Member

Re: Cisco 2600 Sip-UA over a NAT?

If i apply an access list won't that just catch any traffic coming from that interface? It's coming from the right interface but because of the NAT the sip messages are displaying the incorrect WAN address.


Instead of

From: <>xxxxxxxxxx@my.wan.ip.address>

I'm observing the SIP Message content from the debug ccsip messages command

I should have mentioned before, its a 2600XM w/ 32MB flash.

Hall of Fame Super Gold

Re: Cisco 2600 Sip-UA over a NAT?

Hi richard,

If is the interface going to the device doing NAT, that is correct. Now, your NAT device should have support for SIP, that is, it should change this address to his own WAN address. Cisco NAT devices, like IOS and the PIX, supports this.


Re: Cisco 2600 Sip-UA over a NAT?

Is this enabled by default, or must it be configured?

For example, I have an 871 doing the NAT, IOS 12.4(6)T6. This has a feature called "NAT - Support for SIP" but not sure if I need to take an extra step to configure it.

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