Hi,
Here is the information from the page
IP Telephony: The Five Nines Story
http://www.cisco.com/en/US/partner/netsol/ns340/ns394/ns165/ns268/networking_solutions_white_paper09186a00800a113e.shtml
"Redundant Cisco CallManagers are not currently stateful. This capability is expected in future releases. Currently, a TCP keepalive is sent every 30 seconds. Three simultaneous negative responses cause failover to the standby, which can initiate new calls. Current calls are not dropped during failover however DTMF signaling info is lost and call may be reset. A failed access device will cause a failover to the redundant Cisco CallManager. Because three keepalives can occur in 61 seconds, the mean time before the 1st keepalive is used bringing the average downtime to 75 seconds. Shortly following this, the redundant Cisco CallManager will have the ability to accept new call requests.
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Regards,
Marc