A. Exchange Server 2007 introduced the concept of cluster continuous replication (CCR) that provided database replication by copying closed transaction logs to a passive server, which then replayed the content into a copy of the database. If the primary failed, the passive copy would become active. Because the previously passive copy could have missed some transaction logs, it would try to contact the old primary to copy any missing data, and also communicate with the Hub Transport servers in the local Active Directory (AD) site, which keeps copies of messages that traverse through the server in their transport dumpsters.
The Exchange 2010 DAG is essentially an evolution of the CCR model. The transport dumpster is still used, but it's used more efficiently. With Exchange 2010, the Hub Transport receives feedback on the state of replication, which allows it to delete messages from the transport dumpster that have been replicated to all database copies. This allows the transport dumpster to be smaller, because it's only storing messages that haven't yet been fully replicated.
Additionally, because DAGs can span multiple AD sites, when a copy is activated, the Hub Transport servers in both the original and new sites are queried for possible lost traffic.