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

[Gordon Henderson]

On Thu, 16 Oct 2008, 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.

Is it? I've always needed a STUN server in Grandstreams behind a NAT 
router - maybe the NAT router has a (shock horror!) working SIP ALG in it? 
(What make is it?)

> 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?

Does the Cisco support STUN?

Gordon