Forum Discussion
Exchange 2019 and IP-less DAG - how does failover work?
Hey all. We currently have 2 Exchange 2013 servers (main and DR) in a DAG. We have a DAG DNS A record with the IP address of the MAPI interface on our main Exchange server. Our mail.ourdomain.c...
Generally you would have some sort of load balancer in the network path that accepts connections to the DNS namespace (ie webmail.domain.com; autodiscover.domain.com; smtp.domain.com etc) and detects the endpoint is down and redirects traffic to the server that is up (or you can control this during planned outages).
If you don't have a load balancer, you can also use DNS round robin, where you have a DNS record for each Exchange server that corresponds to the namespace required (ie webmail.domain.com; autodiscover.domain.com; smtp.domain.com etc) and clients will automatically try the other DNS record if the first one fails.
The final option is manually change the IP address of the DNS record when a failover is required, although I wouldn't be recommending that for obvious reasons.
If you don't have a load balancer, you can also use DNS round robin, where you have a DNS record for each Exchange server that corresponds to the namespace required (ie webmail.domain.com; autodiscover.domain.com; smtp.domain.com etc) and clients will automatically try the other DNS record if the first one fails.
The final option is manually change the IP address of the DNS record when a failover is required, although I wouldn't be recommending that for obvious reasons.
- Thanks for the reply Dan_Snape.
We don't have a load balancer. Seems load balancers are separate devices and not just another Windows server. We are a small organisation and can't afford one of these let alone two that would ideally be required.
With DNS round robin would it always try a particular IP first? Our backup server is at a remote location which is on a slower link than what clients use to connect to the main server. We would want to ensure that the main server is always preferred. Also how does the client determine which dns entry to use first, is it alphabetical?
Even though it is more complex we may just have to set it up like our existing 2013 servers, with an IP-full dag. It works for our environment.- The Exchange DAG really shouldn't have anything to do with failover or HA of client connectivity or message flow. All the DAG is designed to do is replicate databases and make sure they fail over correctly. If you're using the DAG for anything else you are most likely in an unsupported configuration (it may work, but it may change in the future and cause your configuration to fail).
DNS is not designed to do any kind of load balancing, so if you have 2 DNS records, pretty much 50% of connections will go to one and 50% will go to the other. You can do some hacks and get something like load balancing, but the results are variable and prone to change at any time which could cause issues down the track.
Depending on what is servicing DNS, you may be able to use weighted round robin. Details can be found here for DNS on Server 2016 https://learn.microsoft.com/en-us/windows-server/networking/dns/deploy/app-lb
