External calls drop during server failover

Unanswered Question
May 28th, 2008


We have an install including 2 CUCM 5.1 servers (Pub and Sub), about 80 phones, and one voice gateway (with 1 PRI, and 1 T1 CAS circuit). All endpoints register with the Sub as their primary server with the Pub being the backup. Almost all incoming/outgoing calls go through the PRI. The VG is a 2851, running 12.4(15)T5, and is configured in MGCP mode. When doing our normal testing at cut-over time, we noticed that any established external calls got dropped when the Subscriber was taken down. All endpoints would register properly with the Pub, including the MGCP gateway and the PRI/T1 CAS ports, but active calls were not preserved. According to our Cisco SE, and some literature I've seen on CCO, this should work as advertised, but it's not. I believe I have seen this behavior on other installs as well, but thought that was normal until now.

I am attaching the "show run" of the 2851 gateway. If anybody has any ideas as to what might be the cause of this, I would appreciate your feedback.


Joseph E.

I have this problem too.
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Chris Deren Wed, 05/28/2008 - 09:45

You are correct the call should be preserved on a PRI in case of CM server failure. I've seen this issue multiple times with buggy IOSes, I'd suggest opening a TAC case.


jessakhanian Wed, 05/28/2008 - 13:38

Thanks Chris. That's what our Cisco SE is suggesting as well. According to him, our configuration is correct. I was hoping, I guess, that someone else might have run into this, specially if it was a configuration issue (maybe we were misssing a line of code, or something like that), but it doesn't seem to be the case.

Thanks again,


ecornwell Mon, 07/07/2008 - 16:16

Hi Joseph,

Did you ever get a resolution to this? We a gateway with 2 PRI's and a remote gateway with a single PRI and I've never seen either of them keep up an active call.



weavermj1982 Thu, 01/14/2010 - 07:28

Hi Joseph,

We are running a CUCM 6.1 install with one pub and 2 subs. We have the same issue whereby in progress calls from an IP phone registered to a sub as the active CM drop around 15 seconds after the subscriber is failed over to the pub.

Did you ever get a resolution to this problem? I'd be interested to hear from others who have the same problem, as I don't think it's right that active calls should be dropped.



weavermj1982 Thu, 12/09/2010 - 10:46

Thanks Tarik, sadly this did not work and the audio dropped on the outbound call leg 15 seconds after the connection to the publisher (primary CM) was lost.

Do you have any expertise/experience in this area? I'd really like to get outbound call preservation working for piece of mind should any 999 calls be routed out to the PSTN during a failover period.


tarik.gad Tue, 12/21/2010 - 00:08

you must enable the Allow Peer to Preserve H.323 Calls parameter from the Service Parameters window

weavermj1982 Wed, 02/09/2011 - 11:22


I finally got around to implementing this today and can confirm that I now have call preservation working across our cluster. Thank you for your pointers as this is an issue that we have been wanting to fix for a long time.

Best wishes,



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