Exchange Migration query

Question

I have a few questions on the Exchange 2016 migration

The best practice is to change mail flow control from old exchange to the new exchange as soon as possible.I assume that means Clients / Applications can still connect to old exchange, but any external inbound / outbound emails should go via the new exchange servers? Will clients keep connecting to the old exchange until we migrate their mailboxes OR when we change the Internal URL?

For DAG as it will have an IP based configuration the 2 nodes will be active and passive right so what happens when the active node goes down . What and how changes needs to be made for mailflow to flow from passive node when the active node goes down

Answer 1

You should shange the URL to new server. For mailboxes in old server, the new server(newer version) will proxy the request to the old Client Access server that exists within the mailbox’s local site.
You can read the blog for better understanding:https://techcommunity.microsoft.com/t5/exchange-team-blog/client-connectivity-in-an-exchange-2016-coexistence-environment/ba-p/603945
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.

IF you are having two members' DAG, you must have assigned a Witness server, where is the Witness server located?
DAGs with an even number of members use the failover cluster's Node and File Share Majority quorum mode, which uses an external witness server that acts as a tie-breaker. In this quorum mode, each DAG member gets a vote. In addition, the witness server is used to provide one DAG member with a weighted vote.
If it didn't lose quorum, the database copy will activate on the working server and mail flow should go through that server automatically.

Answer 2

Having done several Exchange migrations myself: You should read these documents. One important thing to do is to create a common name space which does not refer to server names, such as mail.domain.no.

Migrating to Exchange Server 2016
https://practical365.com/exchange-server/migrating-exchange-server-2016/

There's an excellent 3.15 hours course at Pluralsight (sign up for a trail)
https://app.pluralsight.com/library/courses/migrating-exchange-server-2016/description

Paul Cunningham has a complete course on Exchange 2016 as well. Cunningham is one of the world's leading Exchange experts. This will answer your questions on DAGs as well. To give you a hint: Usually you need to do nothing. With DAGs in use, they are never completely in sync. However, there are transport resiliency features: Safety Net and Shadow Redundancy
https://learn.microsoft.com/en-us/exchange/mail-flow/transport-high-availability/safety-net?view=exchserver-2019
https://learn.microsoft.com/en-us/exchange/mail-flow/transport-high-availability/shadow-redundancy?view=exchserver-2019