The following steps occur to instantiate a DAG with the addition of the first mailbox server. Exchange has to do quite a lot of work to bring a server into a DAG, especially if it has to install Windows Clustering, so this process does not happen quickly.
1. Exchange validates that the server has the mailbox server role installed and does not host the FSW resource for the DAG.
2. If not present, Exchange installs Windows Failover Clustering on the mailbox server.
3. A failover cluster is created using the name of the DAG.
4. A cluster network object (CNO) is created in the Computers organizational unit (OU) in Active Directory.
5. The name and IP address of the DAG is registered in DNS as a Host (A) record.
6. The mailbox server is linked to the DAG object by populating the MSExchMDBAvailabilityGroupLink property on the server object in Active Directory.
7. The cluster database is updated with information about the databases hosted by the newly added server. These databases remain as standalone active copies until you create additional passive copies through replication to other servers in the DAG.
When you add additional mailbox servers to the DAG, Exchange does the following:
1. Validates that the server has the mailbox role installed.
2. Joins the server to the cluster.
3. Adjusts the quorum model. A node majority model is used for DAGs with an odd number of members, whereas a node and file share majority model is used for DAGs with an even number of members. The quorum model is automatically adjusted as servers join and leave the DAG, including when they are taken offline for maintenance or suffer a failure. The adjustment occurs in the background and does not require any administrator intervention.
4. Links the server to the DAG object in Active Directory.
5. Updates the cluster database with information about the databases hosted by the newly added server.
Although a DAG is visible to the Failover Cluster Manager, you should never attempt to manage any of the resources used by the DAG through this console. If you do, don’t expect much sympathy from Microsoft Support if one of your changes compromises the integrity of the DAG. Exchange stores many important properties for a DAG in Active Directory and the only way that you can manipulate DAG settings properly is through EMC or EMS.
– taken from Exchange 2010 Inside Out Book by Tony Redmond.