Planning for the Hub Transport Role
The hub transport role is a server role responsible for the distribution of mail messages and policy enforcement within an Exchange organization. There must be at least one hub transport role defined for each Active Directory site that contains a mailbox server.
The hub transport role can be added to a server running any other role, with only one exceptions. It cannot be added to a server that is an edge transport server. If software-based network load balancing is used, it also can’t be installed on a mailbox server running a DAG replica.
Several special considerations exist for hub transport servers, as follows:
• Multiple hub transport servers can be established in a site to provide for redundancy ...