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New Member

SIP trunking issue to ITSP

Hi All,

I've been tasked with setting up some SIP trunks in some of our locations to our ITSP and could use some assistance.

Here is our topology.

- A 2800 series router acting as the TDM gateway router.

   - It's connected to the LAN via a GigE interface and connected to the PBX via a serial interface

   - The default gateway for the TDM router is the standby address on the Internet edge routers

- A pair of 2800 series routers for Internet edge & corporate VPN

   -  Router #1 connects to ISP 1 & router #2 connects to ISP 2

- Each 2800 router is running:

   - VPN tunnels back to main office and data centers

   - BGP for peering with the ISP & corporate VPN peer routers

   - NAT for local Internet access (not back hauled to main office or data centers)

   - HSRP

   - No firewall features such as Inspect, CBAC, ZBFW, etc.

- For NAT, each ISP has allocated a /27 block for things that may need a public facing IP address. We are not using an public /24 that gets routed to both ISP.

   - This means our TDM router nats to x.x.x.254 when going through router #1 and nats to y.y.y.222 when going through router #2

- Currently the #2 router is the active HSRP router and is advertising the default route to the #1 router with a more preferred local preference.

- The way our ISTP set up peering for the SIP trunks (for redundancy), they provided us with 2 SIP peering points and allow only the x.x.x.254 IP address of ours to go to one SIP peer and only our y.y.y.222 IP address to go to the other SIP peer.

In order to make some test calls, I've installed xlite on my laptop and have it registered to the TDM gateway router.

When I configure dial peers on the TDM gateway to send calls to the ITSP peer #2, the traffic goes to our Internet edge router #2 and things work just fine.

When I make our Internet edge router #1 router to be the active HSRP router and prefer it for Internet routing (modify the BGP local preference), the calls do not set up.

When I moved routing back to our Internet edge router #2 and tried test calls to both SIP peers, the calls didn't set up or I got some one way audio behavior.

From what I've read, it sounds like NAT is problematic with SIP and if required, I should be using something like SIP inspection, SIP ALG, or CUBE.

At this particular point, I'm curious if this is even a desirable topology. I'm trying to make do with what I've been provided with but the call testing has me wondering if there is a more elegant solution to provide the SIP trunking to both ITSP peers and have less headaches with things like HSRP, BGP, and NAT in the equation.

If posting configs would help, I can provide.

Thanks in advance!

3 REPLIES
VIP Super Bronze

SIP trunking issue to ITSP

SIP and NAT are not best of friends. To make them work together you need to use SIP ALG within the cisco IOS gateway. This has been discussed a few times here

If you search the forum you should get more threads on sip and NAT..The link below discusses the SIP NAT ALG feature.

http://www.cisco.com/en/US/docs/ios-xml/ios/ipaddr_nat/configuration/15-mt/nat-tcp-sip-alg.html#GUID-1055ECB2-F871-42C7-895D-30554CCEB658

The link below discusses how to configure HSRP for CUBE

http://www.cisco.com/en/US/products/sw/voicesw/ps5640/products_configuration_example09186a0080b40d82.shtml

Looking at your topology, BGP shouldnt be a problem hovwer NAT and HSRP could be...

We can look at logs and your config to see where things are going wrong..

So do a test call and send me debug ccsip messages.

Also attach the sh run from the gateway the test is done..include the calling and called number

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New Member

Re: SIP trunking issue to ITSP

I've read up on SIP ALG and CUBE and at this point am still looking for topology best practice advice.

Currently, we have just the 3 routers- One as the TDM gateway and the other 2 for Internet edge/NAT/site-to-site VPN.

Since NAT & HSRP are required on the Internet edge/VPN routers, does that mean I really should have another (4th) router to server as a CUBE?

Should the CUBE be connected to the Internet with a public IP so it does not have to route through the other routers doing NAT & HSRP?

Can I configure the existing TDM router as a CUBE as well or best to have it as 2 routers, each doing their respective function?

From what I've researched, it seems like keeping the VoIP functions and data connectivity functions on different devices that are independent of one another may be the better way to go but I may be wrong.

Thanks for the help.

VIP Super Bronze

SIP trunking issue to ITSP

Since NAT & HSRP are required on the Internet edge/VPN routers, does that mean I really should have another (4th) router to server as a CUBE

No, CUBE feature is embedded in IOS. You can use your existing gateways. You will need to purchase CUBE licenses though...

Should the CUBE be connected to the Internet with a public IP so it does not have to route through the other routers doing NAT & HSRP?

From your topology, this might be a better approach. This will eliminate the need for NAT which can create issues with SIP

Can I configure the existing TDM router as a CUBE as well or best to have it as 2 routers, each doing their respective function?

Yes you can. Data and Voice can reside on the same traffic. There is no justification to buy and expense router just for voice..

Please rate all useful posts

"opportunity is a haughty goddess who waste no time with those who are unprepared"

Please rate all useful posts "The essence of christianity is not the enthronement but the obliteration of self --William Barclay"
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