SRST question

Unanswered Question
Mar 28th, 2008

We have a CCM (hq and branch site) with one gateway setup in each location as h323.

If CCM goes down and phones register to gateways for SRST, I need some help understanding the call flow when CCM is down.

Example, in normal condition our voip dial-peer points the 4 digit destination-pattern to CCM. But when CCM goes down and those phones register to gatekeeper, how do incoming calls know to "not" go to CCM? Is it because dial-peer will fail to CCM and phones that registered now have "virtual" pots dial-peers for each one that will allow calls to be routed appropriatly to those phones?

What about if one uses CME for SRST. Phones/dial-peers are already created. So how would calls correctly get routed to CCM in normal condition and only to CME registered phones only when CCM is down? Is that becuase even though the ephone ephone dn's were created (which creates virtual pots dial-peers), since the phones are "not connected" the call will go to ccm via voip dial-peer that points to it?


I have this problem too.
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mattcalderon Fri, 03/28/2008 - 05:24

Essentially what happens is that the phones lose communication with the call manager and the phones then go into fall back mode. The phones can be configured with a SRST reference in call manager or use the default of their default gateway.

Once connectivity to the ccm is lost, the phones then register with the router and use the routers dial-peers to make outbound calls.

The router knows that if a voip dial-peer is no longer reachable, it will discontinue its use. You could make 4 digit dialing transparent to the users while in fall back by using num-exp, trnaslation-patterns ect...ect..

Once the phones are in fallback, there are configurations that are in place to do the tasks of either taking incoming calls or placing outbound calls.

The purpose of SRST is for short term connectivity loss to the call manager. If you have phones that are registered to the call manager, there would be no point to have CME configured on the remote router. Just use SRST.

Here is an interactive SRST link:

victor2008 Fri, 03/28/2008 - 06:16

My question really boils down to inbound calls. The telco is sending 4 digits (every phone has a DID). So when ccm is up, the 4 digit pattern is sent to CCM via dial-peer (again gateway running in h.232 mode). But when CCM goes down, how do those incoming calls automatically get sent to the phones (from a call routing perspective, I already understand srst phone registration). Do the calls get sent to the phones, because phones create a "virtual" dial-peer (and since the 4 digits coming in always match a phone's DID)?

What about if your running CME for SRST? The ephone and ephone-dns' are created from day one, so those "virtual" dial-peers would of already been there? How does call routing know that the ccm dial-peer is primary? Is it becuase the the "virtual" dial-peers for CME phones by default has a preference of 1? And dial-peer for CCM is preference 0


jamesfang98 Fri, 04/18/2008 - 10:57

Worth to point this out:

After phone fail to CCM and then register to VGW, VGW will query phone configuration and also build the database of all newly registered phone configuration.

I belive, this means all phones ext. numbers (say 4 digits) will be recognize by VGW (through 'virtual' dail peers) and in turn vgw will route the incoming calls properly.

If you use TranslationPattern in CCM to manipulate the incoming DID number, then you need to have similar way in VGW using 'num-exp', 'tranlation rule', etc,.

Hope the above will explain the questions.

Please let us know your test result.


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