Cisco WS C4224V 8FXS - 200Mbps Ethernet Switch Software Configuration Manual page 205

Chapter 8 Configuring Survivable Remote Site Telephony
Overview of Survivable Remote Site Telephony
of three entries. When a secondary Cisco CallManager is not configured, the
SRST Catalyst 4224 is listed as the stand-by Cisco CallManager during normal
operation hosted by a single Cisco CallManager.
When the WAN link fails, calls in progress are sustained if possible for the
duration of the call. Calls in transition must be attempted again after the
Cisco IP Phones re-home to the local SRST Catalyst 4224. The telephone service
is unavailable from the time the connection is lost from the remote
Cisco CallManager until the Cisco IP Phone is re-homed to the Catalyst 4224
with the SRST feature.
The time taken to re-home to the remote Cisco CallManager depends in part on
the keep-alive period set for Cisco CallManager. Typically, it takes three times
the keep-alive period for the phone to discover that its connection to
Cisco CallManager has failed. The default keep-alive period is 30 seconds.
If the phone has an active standby connection established with the SRST
Catalyst 4224, the fallback process itself takes 10 to 20 seconds after the primary
Cisco CallManager has failed. An active standby connection to the SRST
Catalyst 4224 exists only if the phone has a single Cisco CallManager in its
Cisco CallManager list. Otherwise, the phone will maintain a standby connection
to its secondary Cisco CallManager.
If the phone has multiple Cisco CallManagers in its Cisco CallManagers list, it
must search through the list of secondary and tertiary Cisco CallManagers, which
means the fallback time increases. The phone attempts to connect to each of its
alternate Cisco CallManagers in turn before attempting to connect to the SRST
Catalyst 4224 as a last resort.
Each Cisco CallManager connection attempt will take about one minute. A
message is displayed on the Cisco IP Phone indicating that the Cisco IP Phones
are in Cisco CallManager fallback mode.
These telephone services are restricted to those Cisco IP Phones that are
supported by SRST Catalyst 4224.
The Cisco IP Phone 7960 and 7940 display the message "CM Fallback Service
Note
Operating" at the bottom of the screen. The Cisco 7910 IP Phone displays the
message "CM Fallback Service" for 5 seconds every 30 seconds. The message is
brief because the Cisco IP Phone 7910 has only a two-line display area and the
display panel is also used to display the telephone number.
Catalyst 4224 Access Gateway Switch Software Configuration Guide
8-5
OL-2031-02