A. In Exchange 2007, a cluster Mailbox server couldn't host any other roles. With Exchange 2010, however, you can have highly available Mailbox servers in a DAG also running the Hub Transport, Client Access, and Unified Messaging server roles.
Consider what happens when a Mailbox server fails. A copy of the database is activated and checks with the Hub Transport severs in the site for any lost traffic (because all messages are routed through Hub Transport servers). If the Mailbox server is a Hub Transport server and routes its own traffic, when that server is unavailable, so is its transport dumpster with the required information.
To address this, if the Hub Transport role on a server receives a message to route for a database that's active on the local box, it will attempt to reroute the message to an alternate Hub Transport server in the same site to retain the resiliency the transport dumpster provides.
