Where did the hub transport server role go in Exchange 2013?

By Devin Ganger,

Though the hub transport server role was officially retired in Exchange 2013, the modified server role architecture actually improves transport. Here's how.

What do you think is the most important Exchange Server role?

If you took a survey of Exchange administrators, the most popular answer would be the information store, as most time is spent on mailbox database design during Exchange deployment planning. Client-access namespaces and certificates also eat up a big chunk of the pie, while client configuration and third-party accessories are in the mix as well.

Those are all well and good, but in my mind, the real secret sauce is the hub transport server role. Transport actually moves messages from sender to recipient. Without transport, you don't have email. Instead, you have an anal-retentive database and a website with multiple personality disorder. The simple fact that transport works most of the time is evidence as to how awesome it really is.

Transport is still under the hood

Exchange Server 2013 only has two roles: the mailbox server role and the client access server (CAS) role. So, if transport is so important, why isn't the hub transport server role still included?

If Mark Twain had been an Exchange administrator, he might have said, "Reports of the hub transport server role's demise have been greatly exaggerated." While Exchange 2013 transport has gone underground, you can see that it's far from gone.

In accordance with the design principles of Exchange 2013, the hub transport server role was broken up into multiple pieces and placed where they would do the most good.

Transport Service

The classic hub transport service that we all know -- and some of us love -- lives on in the mailbox server role. The Transport Service (Microsoft Exchange Transport in Figure 1) handles all inbound Simple Mail Transfer Protocol sessions from other servers, all outbound SMTP sessions with down-level Exchange hub transports and also outbound SMTP sessions to both Exchange 2013 mailbox and CAS servers.

The Transport Service handles all content conversion, transport agents and the categorizer. It is also the only one of the three transport services that will queue mail, meaning that only mailbox servers can be bridgeheads for Send connectors.

Although the Transport Service lives on in the mailbox server role, there is no guarantee that outbound messages from active mailbox databases on a given mailbox server will be sent through the Transport Service on the same server.

Up to this point, this behavior is the same as in previous versions of Exchange. What's different, though, is that the Transport Service is a valid delivery target for submitted messages from other Exchange 2013 mailbox servers in different sites -- a distinct difference from previous versions.


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