Forum Discussion
Exchange Server 2019 HA & DR - Design
Hello Dan,
Thanks for your reply with advice and information. I have attached a screenshot of the dag design where i have 6 servers (3 in each subnet).To begin i will only keep 2 servers in each site because i can avoid Witness Server.
You were right about true HA and in our scenario we don't have that one. We have 3 offices and each office has a separate subnet. If I keep witness server in third office, the problem is, it has connection in only one main office and not the second office where I want to keep second set of dag servers for DR. so to avoid witness i will keep even number of nodes in both these offices and they are connected by dark fibre.
The rest I will use single URL in all nodes and if you have any pointers to test main scenarios before migration I would appreciate it.
You'll need a witness server if you have more than 1 Exchange server total in your environment, and it comes into play when you have an even number of servers up and running (so whether you put 3 or 2 servers in each site doesn't matter because both numbers lead to a an even number of Exchange servers in the organisation). If you can't get independent links to a 3rd site, then you should put the witness server in the site you want the Exchange servers to remain up and running in the event a network failure means a whole site goes offline (ie if the witness server is in site A and site B goes offline or network isolated, then the servers in site A will stay online). The problem is that if Site A get's isolated/offline, because the witness server is in site A, the servers there will stay online and the servers in site B will dismount their DB's (the majority is still in Site A: 3 servers plus the witness is a majority) causing an outage.
If HA is critical in all scenarios, you'll need to upgrade your network so that the 3rd site has an independent link to Site B and thus it will be able to tell which site is actually isolated/offline and mount the DB's in the site still running.