Exchange 2007 used LCR, SCC, CCR and SCR for high availability and site resilience of the mailbox databases. LCR database is replicated to another disk on same server, when hardware failure the mailboxes will not be available as the replica of the mail database are stored in the same hardware with different drive. SCC is cluster configuration in which the mail database are stored in shared drives, if failure of the shared drive occurs, the availability of the mailbox will be affected. CCR contain only two members, one active and one passive node. In SP1 new feature called SCR by which the database from the primary site can be replicated to disaster site and achieve site resilience.
DAG is the most significant feature that has made available in Exchange 2010 which includes many of the limitation of the high availability features that were available with previous version of the exchange.CCR used onsite replication and SCR used for offsite replication has been combined together to from DAG in exchange 2010. Microsoft defines DAG as a group of the up to 16 mailbox servers that host a set of databases and provide automatic database-level recovery from failures that affect individual database.
A DAG is a boundary for mailbox database replication, database and server switchovers, and failovers, and for an internal component called Active Manager. Active Manager is an Exchange 2010 component which manages switchovers and failovers that runs on every server in a DAG.
Any server in a DAG can host a copy of a mailbox database from any other server in the DAG. When a server is added to a DAG, it works with the other servers in the DAG to provide automatic recovery from failures that affect mailbox databases, such as a disk failure or server failure.
DAGs leverage a feature of Exchange 2010 known as incremental deployment, which is the ability to deploy service and data availability for all Mailbox servers and databases after Exchange is installed. After you've deployed Exchange 2010, you can create a DAG, add Mailbox servers to the DAG, and then replicate mailbox databases between the DAG members.
A DAG is initially created as an empty object in Active Directory. This directory object is used to store relevant information about the DAG, such as server membership information. When an administrator adds the first server to a DAG, a failover cluster is automatically created for the DAG. This failover cluster is used exclusively by the DAG, and the cluster must be dedicated to the DAG. Use of the cluster for any other purpose is not supported.
During creation, the DAG is given a unique name, and either assigned one or more static IP addresses or configured to use Dynamic Host Configuration Protocol (DHCP). You can specify a single IP address or a comma-separated list of IP addresses by using the DatabaseAvailabilityGroupIPAddresses parameter.
New-DatabaseAvailabilityGroup -Name DAG1 -DatabaseAvailabilityGroupIPAddresses 10.0.0.25,192.168.0.25
Every DAG is a Windows failover cluster. Failover clusters use the concept of quorum. Quorum is not a new concept for Exchange 2010. Highly available mailbox servers in previous versions of Exchange also use failover clustering and its concept of quorum.