SIP Natting

[pmoore]

Testing SIP trunking behind the SRP521 at present, SIP ALG is presently disabled. Using the POTs port for the analogue fax line but have a SIP trunk on port 5060 sitting behine the unit. The SRP is doing some sort of SIP fix up as it changes the outbound port from 5060 to 1024, hwere before It was not, Havn't had a change of config so just wanting top know what the default behaviour was and why the unit would start changing ports.

My Trunk setup is set as a static IP with natted IP so we are always sending to 5060 and via header will always contain the private IP,

Have a copy of the traces if required.

[pmoore]

As an update , when the SIP ALG parameter was toggled the behaviour reverted back to passing 5060 through the device with no port change which is the desired behaviour.  having the port change randomly is not good for static non registering trunks.

[pmoore]

Any thoughts Guys ?

[Andrew Hickman]

Hi Phil,

Could you provide the traces please?  Are you saying that port handling changes randomly, or is just different depending on whether ALG is enabled or not?

Andy

[pmoore]

Hi Andy

Traces sent to your email Andy, the behaviour I saw was the SIp ALG was turned off with a IP PBX sitting behind the 521, it was working on 5060 no problem at all for a number of days then inbound calls started failing as the 521 had started changing 5060 on the inbound side to 1024 on the public side,  As this connection was a staticly mapped port 5060 this SIP fixup was breaking inbound calls due to the 521 wanting 1024 on the public for the natted device.

Once I toggled the SIP ALG on the from off to on and then back to off again , the issue resolved itself as it once again passed 5060 transparently through. the traces should show it pretty clearly.