[asterisk-users] SIP: difference between Grandstream and Cisco when behind NAT

[peder at networkoblivion.com]

You generally don't need to enter the public IP of the router into the 
Cisco, just setting nat_enable to 1 is almost always sufficient.  * is 
smart enough to realize that the IP of the packet is the public IP of 
the phone.

Tony Mountifield wrote:
> I have used Grandstream phones for years, and have just started testing
> a Cisco 7940 (with SIP firmware 7.4). I have found something puzzling
> and don't know whether it's just a limitation or something I haven't
> done correctly.
> 
> The Asterisk server is directly on the Internet with a public IP.
> The phones are on a private LAN with a NAT router to the Internet.
> The sip.conf entries for both phones say nat=yes. For the Grandstream,
> this is always sufficient to make it work properly with Asterisk,
> even though in the Grandstream config I have "NAT traversal: no" and
> leave "Use NAT IP" blank. All the clever stuff is done automatically
> by Asterisk.
> 
> However, with the Cisco, that doesn't seem to be the case. I have found
> it necessary in the SIPDefault.cnf file to set "nat_enable: 1" and
> then specify as nat_address the public address of my router.
> 
> Is this normal? What is different between the Grandstream and the Cisco?
> Is there any way to avoid having to program the external address into
> the Cisco when it is behind NAT?
> 
> Thanks in advance for any advice.
> 
> Cheers
> Tony