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

[Tony Mountifield]

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
-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
Play: tony at mountifield.org - http://tony.mountifield.org