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New Member

SIP trunks and SBC

Hi all

I have a customer who has a 4.1(3) CUCM Cluster and needs to interface with a Mitel 3300 across a private P2P WAN for the purpose of initiating and transferring calls (in both directions).

Both endpoints support SIP and the functions required should work however the WAN has a FW at either end with NAT configured either on teh FW or WAN Router.

I understand that I need to use a SBC to manage this NAT traversal

Could someone please explain

1) What I need?

2) And what topology this should be deployed in?

I have attached a basic schematic that illustrates my situation.

Any help would be greatly appreciated,




Re: SIP trunks and SBC

Id probably put in a Cisco IOS gateway to handle the SIP traffic. I dont think the native SIP from CUCM 4.x and Mitel will work. It might? but you would have to test.

If SBC puts up the IP PTP (which really all it is) SIP is the protocol to transport the voice.

The question remains, IN CUCM, you will make a SIP, RPs to use the SIP trunk, then send it off to the Mitel 3300. SIP in CUCM is very basic.

Open SIP up on the firewall 5060 and 5070 (I think) Point each system to each other and try a call.

I dont think this a SIP trunk for PSTN service or is it?

New Member

Re: SIP trunks and SBC


Thanks for the comments.

the feedback from my Cisco SE is that it "should" work, however the usual testing caveats apply.

the SIP trunk is not for PSTN calls and we intend to have a prefix that will be used to initiate the SIP trunk.

Would the SBC need to sit in a DMZ on each FW or could it sit on the inside?

also would it be required to have one per site?


Re: SIP trunks and SBC

What Im trying to figure out is the actual pipe between site a and site b. To me, I would just buy a PTP pipe. Have (2) routers for IP traffic configured and then layer my SIP into that traffic.


Whether is some magic SBC SIP trunk service PTP (maybe I have not heard of this from them (ATT)) it would sit outside the firewall. Poke holes in your firewall to allow SIP through.

New Member

Re: SIP trunks and SBC

the pipe between the site is a point to point private 100 Mbps Ethernet circuits addressed with a /30

However due to overlapping IP Addresses at both locations NAT will need to be implemented.

Re: SIP trunks and SBC

I think you should be ok. I would go ahead and just open up the SIP ports you need and try it. In the CUCM 6.x you can open up RTMT and watch the SIP traffic live coming/going on the server. Works out great for fixing issues.

On the Mitel, I cant remember on the 3300 if there is a monitor as well, but it will be handy to see that traffic.

In the beginning, I would open all ports to/from that IP address on your FW just to be safe. Then once you get it to work, lock it down to only TCP/UDP ports you need. (probably 5060 and 5070 I assume)

New Member

Re: SIP trunks and SBC

Thanks again

So do you think that a SBC will not be needed in this instance?


Re: SIP trunks and SBC

In the past here is how I have done PTPs with SBC.

They land the circuits on each end and give me the configurations. I install my routers and configure the physical layer so they are up/up. Then I add in my transports (IP) to take the traffic from point A to point B and vice versa.

Once that is up, then I can push any protocol I want through the pipe. I can even add Layer 3 QOS and prioritize the traffic because it's a PTP.

This is based off the assumption the PTP is what I described above. If its the product called IP Flex which provides SIP PSTN connectivity (ATT), then that is different.

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