I am trying to plan my second Exchange 2013 install, it will be a single server hosting both the mailbox and CAS roles. The first exchange server is working just fine. What I want is that the second server can take over if the first site gets struck by an asteroid.
I put together a real quick diagram to help you see what I am thinking of, I got this basically from a couple of cbtnuggets videos I've been following. Let me know if clients external and internal can failover to either of the CAS boxes. I will eventually create a DAG between the two, but I'd like to setup the CAS failover first.