Why SIP trunk is needed !

Unanswered Question
Jun 3rd, 2010

Hi experts.

I am confused so pls follow my wordings. Lets say, i have a private network and i have SSCP phones every where. Is there ever a need to establish SIP trunk between any routers ?

I was just going through cisco gateway/gatekeeper to clear my concept for the word "trunk". There is chapter 4, they have given the example, where analog phones are connected to pbx and pbx is connected to GW-A. GWA is connected to GW-B over IP network. GW-B is connected to PBX which in turn connected to analog phone like below


Now its written that, a SIP trunk is made between GW-A and GW-B. Why do i need a sip trunk here ? cant i just forward the call on IP network as IP packets ?

Am i missing something here ?

I have this problem too.
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Jonathan Schulenberg Thu, 06/03/2010 - 04:38

The call agents on the gateways must know how to reach one another. The call occured using a telephone number, not an IP address so normal IP routing protocols are not useful for the call setup. Gateway A must be configured to contact Gateway B when the call is destined to Analog Phone 2 in your example.  Likewise Gateway B must be configured to properly receive the call when it is from Gateway A and then forward it on to the PBX. The Cisco IOS-based call agent does this using incoming and outgoing dial-peer statements. The signaling between the gateways can be SIP or H.323.

Jonn cos Thu, 06/03/2010 - 04:59

Jonathan, i didnt get it. If you just see my scenario, i thought just defining dial peers would be enough. Kindly note that both these routers are under my configuration, do i still need a sip trunk ?

Jonathan Schulenberg Thu, 06/03/2010 - 05:04

Dial peers are sufficient to form the SIP connection between the two routers if properly configured. Just because you don't have a service provider involved doesn't mean it isn't a trunk.

Jonn cos Thu, 06/03/2010 - 09:22


If in my scenario, i dont configure sip trunk, and just configure dial peers so you mean to say it wont work ?

Jonn cos Thu, 06/03/2010 - 21:52

Dear Jonathan,

I am completely lost with your statement :-) (just kidding). I know there is a cisco press ebook for Sip trunking, but i dont think its of my level right now. But i really didnt get your statement, that dial peers are my sip trunk. If possible, can you pls explain it and also provide some reference for a newbie like me to get better understanding of it pls ?


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