Hi Dave,
There are some good design ideas for EM Redundancy in this clip. These are the "best" tips that I have seen so far;
Beginning with Unified CM 6.1(2), EM functionality is no longer dependent upon the
publisher server because the required database writes can be done on subscriber nodes. If the publisher is unavailable, EM logins and logouts are still possible.
From a redundancy perspective, the following three component levels of redundancy must be considered for full EM resiliency:
• Cisco CallManager Cisco IP Phone Services
As previously discussed, the Cisco CallManager Cisco IP Phone Services is required for providing the IP phone services menu to the phone. The menu of subscribed phone services is displayed when the user presses the services button on the phone. This is necessary so that the user can select the EM phone service. The node to which a phone points for the Cisco IP Phone service is determined by the URL Services parameter. Only a single service URL can be configured per phone. By default this parameter points to the publisher node.
• EM IP phone service
The EM IP phone service is the service that is selected by the user from the IP phone services menu (or, alternatively, from a services line button) in order to log in or log out of a phone. This phone service points to the Cisco Extension Mobility Application service running on a particular
Unified CM node. As indicated previously, the Cisco EM Application service provides the interface between the user (or phone) and the Cisco Extension Mobility service. The EM IP phone service can point to only a single IP address or host name.
• Cisco Extension Mobility service
The Cisco Extension Mobility service is required for EM login and logout. This service takes user credentials from the Cisco EM Application service and then writes to and reads from the local Unified CM database.
In order to provide redundancy for the Cisco CallManager Cisco IP Phone Services (or URL Services) and the EM phone service components, Cisco recommends using the Server Load Balancing (SLB) feature available in Cisco IOS to front-end multiple Unified CM nodes. The Cisco IOS SLB feature
provides a virtual-IP-to-real-IP address mapping, that front-ends the real IP addresses of the Unified CM nodes. The SLB feature can be configured to monitor the status of multiple nodes, automatically redirecting requests during failure events. By using the SLB virtual IP address (or DNS hostname) for the URL Services and EM IP phone service, you can ensure that both components are still available during any node failure and, therefore, EM login and logouts will continue.
As an alternative to Cisco IOS SLB, DNS can be used to provide redundancy for these components. To use DNS as a redundancy mechanism, a DNS A record should be configured for each Unified CM subscriber node with the same fully qualified domain name (FQDN) or host name so that a DNS query
for the FQDN or host name will return IP addresses for multiple Unified CM subscriber nodes. Thus, a phone querying the DNS server for the FQDN or host name will receive multiple IP addresses. Given a node failure, these IP addresses can be contacted one at a time in turn until a responding node is found.
The drawback to using DNS for EM redundancy is that a phone must wait for a timeout if a node has failed before attempting to contact the next IP address or node. This timeout can take anywhere from 30 to 45 seconds and will be repeated for each attempt to contact a failed node, therefore significant delay
can occur during node failures before a response is received from a Unified CM node.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/apps.pdf
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