You would need to go to CCM 6.x to have redundancy for EM. Here is the background (there is no way to do this on any CCM version until 6.x)
The configuration database is stored on a publisher server, and a read-only copy is replicated to the subscriber members of the cluster. Changes made on the publisher are communicated to the subscriber databases, ensuring that the configuration is consistent across the members of the cluster, as well as facilitating spatial redundancy of the database.
The publisher server is the only server that has read and write access to the configuration database. When configuration changes are made, other members of the cluster have a read-only copy of the database.
In prior versions of Cisco Unified Communications Manager, subscriber servers in the cluster use the publisher database for READ/WRITE access, and only use the local database for READ access when the publisher database cannot be reached. With Cisco Unified Communications Manager Release 6.0, subscriber servers in the cluster READ the local database. DB WRITES happens in both the local database as well as the publisher database, depending on the type of data. DBMS (IDS) replication is used to synchronize the databases on the nodes of the cluster. When recovering from a failover conditions such as loss of WAN connectivity for extended period of time, the Cisco Unified Communications Manager databases need to be synchronized with any changes that may have been made during the outage. This process happens automatically when database connectivity gets restored. This process may take longer over low bandwidth and/or higher delay links.
Database modifications for CallProcessing
User Facing features can be made on subscribers. These include updates for:
Call Forward All (CFA)
Message Waiting Indication (MWI)
Do Not Disturb Enable/Disable (DND)
Extension Mobility Login (EM)
Monitor (for future use, currently no updates at the user level)
Hunt Group Logout
CTI CAPF status for end users and application users
Cisco IOS gateways can maintain links to up to **two backup Cisco Unified CallManager servers in addition to a **primary Cisco Unified CallManager. This redundancy enables a voice gateway to switchover to a backup if the gateway loses communication with the primary. The backup server takes control of the devices that are registered with the primary Cisco Unified CallManager. The second backup takes control of the registered devices if both the primary and first backup Cisco Unified CallManager fail. The gateway preserves existing connections during a switchover to a backup Cisco Unified CallManager.
When the primary Cisco Unified CallManager server becomes available again, control reverts to that server. Reverting to the primary server can occur immediately, after a configurable amount of time, or only when all connected sessions are released.
Switchback is the process a voice gateway uses to reestablish communication with the primary Cisco Unified CallManager server when the server becomes available again. Switchback can occur immediately, at a specified time after the last active call ends, or after a specified length of time.
MGCP Gateway Fallback
The MGCP gateway maintains a remote connection to a centralized Cisco Unified CallManager cluster by sending MGCP keepalive messages to the Cisco Unified CallManager server at 15-second intervals. If the active Cisco Unified CallManager server fails to acknowledge receipt of the keepalive message within 30 seconds, the gateway attempts to switch over to the next available Cisco Unified CallManager server.
SIP traces provide key information in troubleshooting SIP Trunks, SIP
endpoints and other SIP related issues. Even though these traces are in
clear text, these texts can be gibberish unless you understand fully
what they mean. This document attempts to br...
Please find the attached HTML document, download and open it on your PC.
This provides an easy to use form where you simply answer a few
questions and it will render the proper jabber-config.xml file for you
to copy/paste. There is built in logic to verif...
CUCM Database Replication is an area in which Cisco customers and
partners have asked for more in-depth training in being able to properly
assess a replication problem and potentially resolve an issue without
involving TAC. This document discusses the bas...