DAG with only two Exchange servers (over WAN)?

We are thinking about setting up a DAG consisting of two Exchange machines, with one of them being located off-site with a decent WAN connection. We planned to have one of the serves at the main facility take over the witness role. The main goal is to be able to switch to the remote site in case the main server/network goes down, instead of using the other machine actively (unless that would be better).

With 2 servers down (out of 3), will the DAG even work if the main site goes down?
Will the switchover be automatic, or a manual process in that case?
And finally, how would the fall-back work (as I understand having 1 out of 3 DAG servers on-line will “break” DAG)?  
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rr2rAsked:
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AkhaterSolutions ArchitectCommented:
In exchange 2010 rtm this scenario was not appropriate but is is now valid with the release of SP1.

The failover can be automatic if the server in the main site goes down but if the whole site goes down the dr server will loose quorum and will not mount the dbs so the datacenter switchover is manuel.

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rr2rAuthor Commented:
Going to read through this and see if I can get all my questions answered...
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