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Survivable Remote Site Telephony (SRST) provides Cisco CallManager with fallback support for Cisco IP Phones that are attached to the Catalyst 4224 Access Gateway Switch (Catalyst 4224) on your local Ethernet.
SRST enables the Catalyst 4224 to provide call handling support for Cisco IP Phones when a Cisco IP Phones lose connection to the remote primary, secondary, or tertiary Cisco CallManager or when the WAN connection is down.
This section contains following topics:
Cisco CallManager Releases 3.0 and later support Cisco IP Phones across the WAN that are attached to a branch office Catalyst 4224. Without the SRST feature, when the WAN connection between the branch office Catalyst 4224 and Cisco CallManager fails or when connectivity with Cisco CallManager is lost, the Cisco IP Phones at the remote site are unusable for the duration of the failure.
The SRST feature overcomes this problem, enabling the basic features of the Cisco IP Phones by providing call-handling support on the Catalyst 4224 for its attached Cisco IP Phones. The system automatically detects the failure and uses the Simple Network Auto Provisioning (SNAP) technology to autoconfigure the Catalyst 4224 to provide call processing for the local Cisco IP Phones.
When the WAN link or connection to the primary Cisco CallManager is restored, call-handling capabilities for the Cisco IP Phones switch back to the primary Cisco CallManager. During a failure when the SRST feature is enabled, the Cisco IP Phone displays a message stating that the Cisco IP Phones are in the Cisco CallManager fallback mode and are able to perform limited functions.
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Note The Cisco IP Phone 7960 and 7940 displays the message "CM Fallback Service Operating." The Cisco IP Phone 7910 displays the message "CM Fallback Service." |
Restrictions on the SRST feature are as follows:
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Note First-generation Cisco IP Phones, such as Cisco IP Phone 30 VIP and Cisco IP Phone 12 SP+ are not supported. |
Prerequisites for employing the SRST feature on the Catalyst 4224 are as follows:
For further details, refer to your Cisco CallManager documentation.
The following features are supported on the Cisco IP Phones:
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Note When the primary Cisco CallManager is restored, the displayed message is removed and you can access all the previous functionality of the Cisco IP Phone. |
When the Cisco IP Phones lose contact with all primary, secondary, and tertiary Cisco CallManagers, the phones re-home to the local SRST Catalyst 4224 to provide the call processing capability required to place and receive calls. (Re-homing is a major network change involving moving the local loop termination from one Central Office wire center to another.) The Cisco IP Phone lists the IP address of the local SRST Catalyst 4224 as the default Catalyst 4224 in the Network Configuration area of the Settings menu. This list supports up to five default Catalyst 4224 entries; however, Cisco CallManager uses a maximum 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.
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Note The Cisco IP Phone 7960 and 7940 display the message "CM Fallback Service 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. |
Cisco IP Phones periodically attempt to reestablish connections with the remote Cisco CallManagers. When a connection is reestablished with a remote Cisco CallManager, the Cisco IP Phones unregister from the local Catalyst 4224 with the SRST feature and register with the remote Cisco CallManager. The connection to the primary Cisco CallManager at the remote office cannot be reestablished if the telephone has active calls.
Figure 8-1 shows a branch office with several Cisco IP Phones connected to a Catalyst 4224. The Catalyst 4224 is connected to the remote centralized Cisco CallManager by the WAN and PSTN.
Figure 8-2 shows that the WAN connection to the branch office is down and that the Cisco IP Phones are able to make calls by being connected to the local SRST Catalyst 4224. This Catalyst 4224 acts as a fallback Cisco CallManager. The branch office Cisco IP Phones on the LAN network, connected to the PSTN, are capable of making off-net calls outside the network using the Catalyst 4224s.
When SRST is enabled, Cisco IP Phones do not need to be reconfigured individually during Cisco CallManager fallback mode because Cisco IP Phones retain the same configuration used with the primary Cisco CallManager.
To configure SRST on the Catalyst 4224, use the following steps, beginning in global configuration mode:
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Note In Cisco CallManager fallback mode, huntstop is set by default. (Huntstop disables all further dial-peer hunting for the dial peers associated with the Cisco IP Phone lines if a call fails using hunt groups.) Use the no huntstop command only if you want to disable huntstop. |
To verify that the SRST feature is enabled, follow these steps:
Step 2 Enter the show call-manager-fallback all command to verify that the SRST feature is enabled.
Step 3 Verify that the default Catalyst 4224 IP address on the Cisco IP Phone is the same as the IP address of the SRST Catalyst 4224. Use the settings display on a Cisco IP Phone to do so.
Step 4 Temporarily block the TCP port 2000 Skinny Client Protocol (SCCP) connection for one of the Cisco IP Phones. This forces the Cisco IP Phone to lose its connection to Cisco CallManager and register with the SRS Catalyst 4224. Perform the following:
1. Use the appropriate access-list command to temporarily disconnect a Cisco IP Phone from Cisco CallManager.
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Note The Cisco IP Phone 7960 and 7940 display the message "CM Fallback Service Operating" at the bottom of the display screen. The Cisco IP Phone 7910 displays the message "CM Fallback Service" for five seconds every 30 seconds. This message is brief because the Cisco IP Phone 7910 has a two-line display area and the display panel is also used to display the telephone number. |
2. Delete the access-list command to restore normal service for the phone by entering the no form of the appropriate access-list command.
3. Use the debug ephone register command to observe the registration process of the Cisco IP Phone on the SRST Catalyst 4224.
4. You can also enter the show ephone command to display the Cisco IP Phones that have registered to the SRST Catalyst 4224.
Use the following commands to troubleshoot the SRST feature.
Use the following commands to monitor and maintain the Catalyst 4224 with the SRST feature:
| Command | Purpose |
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Displays the detailed configuration of all the Cisco IP Phones, voice ports, and dial peers of the SRST Catalyst 4224. |
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Displays the output of the dial peers of the SRST Catalyst 4224. |
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This section provides a configuration example for the SRST feature:
Posted: Sat Apr 5 03:59:55 PST 2003
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