The steps below will walk you through adding Databases to an Exchange DAG. The action of adding Database copies to the DAG initiates replication between the servers storing copies of the database, increasing the availability of Exchange. This article assumes that you have already created and configured the Exchange DAG.
Use the Exchange Admin Center (EAC) to add a mailbox database copy:
1) In the EAC, go to Servers > Databases.
2) Select the database that you want add to the DAG, and then click Add database copy.
3) On the “add mailbox database copy” page, click browse…, then select the Mailbox server that will host the new copy of the database, and then click OK.
Optionally, configure the Activation preference number for the database copy.
4) Click More options… to designate the database copy as a lagged database copy by configuring a replay lag time, or to postpone automatic seeding of the database copy. Generally select the defaults here. I generally only configure lagged copies when deploying a third or fourth DAG server. I often select postpone seeding and then later go back in to resume the replication and seed the database. I find this reducing seeding errors because the new database copy status can be replicated in AD before seeding is kicked off.
5) Click Save to save the configuration changes and add the new mailbox database copy.
6) Click OK to acknowledge any messages that appear.
Next, you’ll want to set the DAG’s DAC Mode.