Cisco UCCX with SRST, Hunt-groups and SIP phones, yes it’s possible !

For this first post on the Cisco UC I want to share you the configuration that made me sweat ! I have a customer that has a centralized UC infrastructure with only two CUCM, one UCCX and remote sites with 2901 gateway (IOS 15.4) and some 7821 IP phones. The PSTN is linked to the gateway with an E1 on the local sites and the gateway uses SIP Trunks to the CUCM. There is only one DID on the PSTN acces on each site for the call to the UCCX.

So my challange was to configure the SRST for SIP Phones and find a way to redirect all incoming calls to all the phones (hunt with broadcast) if the WAN link is down and phones registers to SRST. Easy no ? No. (and now if you want the solution jump to the diagram below)
Why ? Because if you configure a hunt group with the same number as the UCCX DN but with lower preference in dial-peer, the hunt group is still local to the gateway and will be preferred than the trunk to CUCM. Worst, if you configure the hunt group after the trunk it would work but after a reload of the gateway it doesn’t work anymore .

I could configure the gateway in MGCP that would be privilegied in normal mode with the hunt group for the SRST, but using MGCP for a PSTN gateway when SIP is almost used every where, I don’t think it’s the best idea ! So what ? Shared-line ? Not supported in SRST, if you try it you would have a dial-peer for each phones with this line and with the dial-peer hunting, the gateway still would select the same phone.

Almost every post on the Net about SRST + Hunt would say to use the magic alias command, but this works perfectly with… SCCP Phones ! And the alias on the voice register pool section seems not working as well. What are the remaining options ? Mhmm maybe a custom TCL script or using the EEM to create dynamically a hunt group when the CUCM Trunk dial-peer is busied-out… yuck !

And finally during a peaceful night the solution appeared, A Cisco gateway does voice routing (yes it seems that it was a strange night). So why not use the hunt group solution not only in SRST but always ? Let’s imagin a hunt-group that would send the call to the UCCX first and then if no answer or error (no WAN) send the call to all phones. And “all phones” is a second hunt group ! Yes it works ! YES !

Here is the solution :

SRST Schema

It could explain more in details,but I think that almost everything is in there ! So here is the basic voice configuration of the gateway :

And that’s it, I can now really have a good peaceful night.

3 Comments

  1. Abollgoog

    I enjoyed reading this post.

    Thanks a lot.

    Reply
  2. Brad

    This is an awesome solution. Thanks for posting

    Reply
  3. Jay

    I see what you did there, nice work mate. Loving the diagram as well.

    Reply

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