How to force users to use their local IM and Presence server in a Cluster over WAN deployment?

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Good Morning Experts,

I am in the middle of designing a Unified Communications solution and am currently looking at the IM and Presence part.  As the title suggests,  I am looking to deploy a Active/Active Cluster over the WAN IM and Presence Service.  My question is this,  we have users at the local sites and i would like to know if there is a way to make them use their local IM and Presence service,  what i dont want is users in a location using a remote IM and Presence Service unless their local one is down?

I have had a look through the Cisco docs but it is not clear to me if this is possible.

Any advice would be appeciated.

Thanks

Neil

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So doing some further reading and googling it appears that i could split out into 2 sub clusters, one at each location with Local failover to ensure users stay at thier local site.  But what i have not been able to glean from the text is if Sub Cluster 1 goes offline completely including local backup would users be able to failover to Sub Cluster 2 at the remote site assuming that IP connectivity was available?