we currently have two exchange 2013 servers at our main site, setup as DAG group with onsite 2008 server as the witness server, which gives us recoverability if something goes wrong with the mailstore, but not if the primary exchange server itself goes down.
now I am being asked to add a 3rd exchange server at our remote site for failover.
but from reading the following technet article on DAG and HA, it would seem you need two exchange servers at each site?
https://technet.microsoft.com/en-us/library/dd638129(v=exchg.150).aspx
what started us down the road to HA is when I rebooted our two exchange servers, one at a time, to apply windows updates, and when the primary server was rebooting email was offline for short time (5 minutes while it rebooted). I think management was under the mistaken impression that DAG=HA, which is not true.
so for true HA, I would need two exchange servers at remote site, plus Windows AD\DNS server at remote site, correct?