[c-nsp] NAT/PAT/Port Forwarding and VOIP

[Eric D. Meyers]

With a NAT translation, is it possible to 'trigger' or pinhole the opening
of ports on a Cisco router?  For example, if an inside host communicates on
a specific port or range of ports can the NAT be configured to open an
inbound port on the outside interface to communicate with that host?



I've got a couple of Siemens 420 IP phones (using HFA rather than SIP, by
the choice of our telephone group) sitting behind a Cisco 827 ADSL router.
The phones are able to communicate with the gatekeeper initially and receive
their configuration, but unable to establish a call unless TCP1720 is
opened.  Static mapping work well, but I'd rather have something a bit more
dynamic.  I've tried to accomplish this under PAT (using the 'overload'
option) and using a NAT pool; no joy on anything I've attempted as yet.



If anyone has assistance to offer it would be quite appreciated.  There
might be some e-beer in it for ya.



Eric Meyers
email:   emeyers at utk.edu