H.323 call survival due to SRST

[bgrunewald]

GWGK training material says that a call from an IP phone through H.323 GW will survive a CCM failover if SRST is enabled.

Can anyone provide a link to an explanation of the mechanism involved? I searched CCO and came up dry.

[Daniel Baum]

What chapter, page is this data? Just want to take a look to verify for the answer.

Thanks

[kthorngr]

From a CCM perspective there is no call preservation for H.323 calls. This is from the 4.1(3) Device Support doc on CCO:

The following devices and applications do not support call preservation in this release:

•Annunciator

•H323 devices

•CTI applications

•TAPI applications

•JTAPI applications

This can be found here:

http://www.cisco.com/univercd/cc/td/doc/product/voice/c_callmg/4_1/sys_ad/4_1_3/ccmsys/a02dvsup.htm#wp1020706

If there are IP Phones at the remote site that have outbound calls through the H.323 GW and there is a WAN failure; the media for the calls will stay up (assuming there are no network issues between the phones and the GW). After ~3 minutes the GW will tear down the calls due to a TCP KeepAlive timeout with the CCM.

You can use the command "no h225 timeout keepalive" on the H.323 GW to keep disable the KeepAlive timeout issue. The calls will then stay up for as long as the call is active.

There are a couple of issues with this though. When a WAN failure occurs the CCM, after ~60 seconds, will have a SCCP KA timeout with the phones. At this point it will unregsiter the phones and tear down the H.323 portion of the call, ie, CCM sends Release Complete.

There is a short window of time (right around 3 minutes) that if the circuit comes back the call could be dropped. For example, TCP will retransmit the Release Complete, if the circuit comes back during this time and the GW receives one of the Release Complete signals it will disconnect the call.

Basically CCM does not support H.323 call preservation today, but it is being looked at for a future CCM release (not sure of the timeframe right now). You can use the "no h225 timeout keepalive" command on the GW to increase the chances of the call staying up.

Let me know if you have any questions.

Kevin

[bgrunewald]

Thanks, that clears it up. I think the book was talking about call preservation when failing over to SRST, but they worded it very poorly.

[kthorngr]

You will have the same issue even when failing over to SRST. CCM doesn't care if you are failing to SRST and the phones won't register to the SRST router until the call is disconnected.

Kevin

[bgrunewald]

It is at the bottom of page 1-64, but my book is a beta I think.

It is the last bullet item in Choosing a Gateway Protocol, Why Choose H.323?