Hi,
here the guide for Cisco Expressway Cluster Creation and Maintenance.
http://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/expressway/config_guide/X8-6/Cisco-Expressway-Cluster-Creation-and-Maintenance-Deployment-Guide-X8-6.pdf
there is a note about the high availability for MRA
High Availability (failover)
High Availability means that if the client fails to connect to the primary server, it fails over to a secondary server with little or no interruption to the service. In relation to high availability being supported on the Expressway for Mobile and Remote Access, high availability refers to the server for the specific service failing over to a secondary server (such as Instant Messaging and Presence), and not the Cisco Expressway-E server itself failing over.
Some services are available on the Expressway for Mobile and Remote Access that are not supported for high availability. This means that if users are connected to the client from outside the corporate network and the instant messaging and presence server fails over, the services will continue to work as normal. However, if the audio and video server or voicemail server fails over, those services will not work as the relevant servers do not support high availability.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/jabber/10_5/CJAB_BK_D6497E98_00_deployment-installation-guide-ciscojabber/CJAB_BK_D6497E98_00_deployment-installation-guide-ciscojabber_chapter_011.html#CJAB_RF_C92F3311_00
and some best prectice
High Availability for Expressway-C and Expressway-E
We recommend deploying Expressway-C and Expressway-E in clusters. Each cluster can have up to six Expressway nodes and a maximum of N+2 physical redundancy. All nodes are active in the cluster. For details about cluster configuration, refer to the Cisco Expressway Cluster Creation and Maintenance Deployment Guide.
Expressway clusters provide configuration redundancy.The first node configured in the cluster is the publisher, while all other nodes are subscribers. Configuration is done in the publisher and automatically replicated to the other nodes.
Expressway clusters provide call license sharing and resilience. All rich media sessions and TURN licenses are shared equally across nodes in the cluster. Call licenses are contributed by the licenses configured on each node.
Expressway-C and Expressway-E deployed as virtual machines support VMware VMotion. VMware VMotion enables the live migration of running virtual machines from one physical server to another. When moving the virtual machine, Expressway-C and Expressway-E servers will maintain actives calls when handling signaling only or when handling both signaling and media. This provides high availability for the Expressway nodes as well as call resilience across Cisco Unified Computing System (UCS) hosts.
The following rules apply to Expressway clustering:
Expressway-C and Expressway-E node types cannot be mixed in the same cluster.
All nodes in a cluster must have identical configurations for subzones, zones, links, pipes, authentication, bandwidth control, and call policy.
Configuration changes should be made only on the publisher node, and this will overwrite the configuration on the subscriber nodes in the cluster when replication occurs.
If a node becomes unavailable, the licenses it contributed to the cluster will become unavailable after 2 weeks.
Deploy an equal number of nodes in Expressway-C and Expressway-E clusters.
Deploy the same OVA template throughout the cluster.
All nodes in a cluster need to be within 30 ms maximum round-trip time to all other cluster nodes. Clustering over the WAN is thus not recommended due to latency constraints.
You must use the same cluster preshared key for all nodes within the same cluster.
H.323 must be enabled on all nodes in a cluster for database replication. H.323 signaling is used for endpoint location searching and sharing bandwidth usage information with other nodes in the cluster.
If mobile and remote access and business-to-business communications are enabled on the same Expressway-C and Expressway-E pairs, the SIP port number used on the SIP trunk between Unified CM and Expressway-C needs to be changed from the default 5060 or 5061.
A DNS SRV record must be available for the cluster and must contain A or AAAA records for each node of the cluster.
Since Expressway-C is deployed in the internal network and Expressway-E in the DMZ, Expressway-C has to be connected to Expressway-E through a traversal zone for mobile and remote access. Expressway-C is the traversal client for mobile and remote access, and Expressway-E is the traversal server. In Cisco Expressway software version X8.2 and later releases, the client and server zones are both called Unified Communications traversal zones. Business-to-business calls require a separate traversal zone, which retains the name of traversal client zone for Expressway-C and traversal server zone for Expressway-E. The traversal server and traversal client zones include all the nodes of Expressway-C and Expressway-E, so that if one of the nodes is not reachable, another node of the cluster will be reached instead.
As shown in Figure 4-16, Expressway-C connects to all servers of the Cisco Unified CM, IM and Presence, and Unity Connection clusters, so high availability and redundancy are preserved across the entire connection path.
http://www.cisco.com/c/en/us/td/docs/solutions/CVD/Collaboration/enterprise/collbcvd/edge.html#pgfId-1076122
The core point for a good deployment is the DNS resolution inside and outside the corporate network.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/jabber/Windows/9_7/CJAB_BK_C606D8A9_00_cisco-jabber-dns-configuration-guide/CJAB_BK_C606D8A9_00_cisco-jabber-dns-configuration-guide_chapter_010.html
hope i dont miss the point
Rate if helps
Sandro
