Forum Discussion
Exchange Server 2019 HA & DR - Design
I personally try to use active/active, mainly because I don't like resources sitting around doing nothing. One thing to be aware of is that true site HA is only available if your sites are also separate AD sites, not part part of a stretched VLAN/single AD site. You will need a witness server if you have more than 1 Exchange server, and this should be at a 3rd site that has independent links to both sites.
If you use active/active, the number of servers in each site is determined by the number of users that the site will be hosting in a failover situation (Microsoft recommend scale out rather than scale up for servers...but they would because that means more licenses!!). There is an Exchange server calculator that can help sizing Exchange environments
https://aka.ms/Exchange2019Calc
https://www.microsoft.com/en-US/download/details.aspx?id=102123
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.
