Intracluster EL-CAC

Hi guys,


I have watched intra-cluster ELCAC videos recently and I have a question regarding who/which CallManager invokes LBM?

I understood that the process is the following:

- CUCM looks for its LBM group

- if no LBM group configured or doesn't answer, it will look for its own LBM process (if up and running)

- if not running, falls back to the SP :"call treatment when no LBM available" (which is "allow calls" by default)


But that does not explain which CUCM goes through this process. Let's have an example to make it clearer:

We have a 3-node CUCM cluster (so 1 Pub and 2 Subs).

Phone1 is registered on Sub1, Phone2 is registered on Sub2.

Phone1 wants to call Phone2, they are not in the same location and bandwith limitations have been defined between these 2 locations.


Which CUCM(Sub1 or Sub2) will invoke LBM to check if there is enough available BW to place the call? In the video, Mark said that the CUCM on which the phone is registered will contact its LBM Group or own LBM. But what if the calling phone and called phone are not registered on the same CUCM ? Will both CUCM (CUCM's calling phone and CUCM's called phone) contact their respective LBM?


This question can be also applied to intercluster EL-CAC because the EL-CAC goes through a SIP trunk that does not really register on a specific node. How does it works? Which CUCM (on each cluster) will look for an LBM to do the math?


I hope my question is clear,




CCIE Collaboration #57461


  • Hi guys,


    Just so you know, I have performed some tests and I confirm that the CPE on which the calling device is registered to is responsible for contacting an LBM node to process the CAC operation.


    I also noted something very interesting:

    Let's say HQ phone1 wants to call SB phone1

    RegionHQ <-> RegionSB : 64 kbps (max audio bit rate)

    But ACPL lists G.729r8 on top of the list! So G.729 should be negotiated

    LocHQ <->LocSB : 30 kbps


    The call should go through, right? Cause 30 kbps of BW is enough to allow a G.729 call of 24 kbps (8kbps of RTP + 16kbps of L4/L3 overhead).


    Well.... CAC denies the call. I went through the LBM logs and CAC says it needs 80k to allow the call to go through (so 64k + 16k overhead)


    So it seems LBM doesn't care about the ACPL involved in your region setup. The reason is probably because the ACPL is only involved during the codec negotiation between phones, which didn't take place yet! CAC comes first and only makes its decision based on the max audio bit rate...



    CCIE Collaboration #57461

  • Sorry,


    My assumption is probably wrong... When I think about it, it's probably because CUCM will never be able to force a specific codec. It's not because I placed G.729 on top of the ACPL that it will be negotiated!

    If the call goes through a CUBE/GW and flows through a VoIP dial-peer on which G.711 is hardcoded, G.729 won't be used!


    So CAC only takes into account the maximum audio bit rate, and it's a good thing cause CUCM cannot predict which codec will be negotiated.




    CCIE Collaboration #57461

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