Wednesday, October 8, 2008

Weird application interactions

Recently we made some changes to our media resource configuration in CallManager 6.x. Specifically, we stopped using DSPs on a 6608 blade in a Catalyst 6500 and started using DSPs in a couple of 2811 routers. Immediately after this change, a couple of users reported that they could no longer make outbound PSTN calls that transit a particular H.323 gateway. The weirdest part was that if they rebooted their 7940 phone, they could make a single call, but all subsequent calls would fail with a fast busy until they rebooted again. Calls through other gateways worked fine.

A ISDN Q.931 debug showed that the second outbound call was setting up as "unrestricted digital" instead of "speech".

I went 'round with TAC about this for a few emails and they were getting ready to blame the telco (?!) when I realized that the thing these two users have in common is VT Advantage (a Cisco desktop video-conferencing application). I asked them to turn off VT Advantage, and presto, all calls now worked.

Upgrading to the latest VT Advantage version solved the problem completely.

Why a change in media resources would affect outbound calling through a single H.323 gateway on users with an old version of VT Advantage is beyond me, but that's my story and I'm sticking to it.


Unknown said...

Interesting enough, the routers where the DSP's are located are running 12.4.20T, which coincidentally happens to be the first version of IOS that supports H.320 Video and IMUXing of b-channels for high bandwidth video conferencing. Previously this function was performed on a standalone H.320 gateway(Cisco IPVC 35xx, or Radvision ViaIP). I thought of this when you said that the only 2 devices that were affected were video enabled. Maybe the router picked up on the fact that these were video-capable devices and that interacted with the video-capable functions in the IOS, somehow?

Jay Swan said...

Thanks for the info--I made a new post today on this.