I am deploying 2 Mailbox servers and 1 CAS server. Should I put the HUB on the CAS server or on the Mailbox server? Also, if I put the a HUB on the Mailbox server, do I put a HUB on each of the mailbox servers or just one? Advantages / Disadvantages to doing that? Does it matter.? All boxes are very beefy, so performance is not a consideration. Thanks for your help. Chad.

Why do you have two mailbox servers? Is it a cluster? if so, you cannot install any other server roles on to a clustered mailbox server. I generally put around 5 thousand users per mailbox server, so i'm curious to know more about your enviroment, but based on the quick bit you've posted, id put the HT on the CAS. But you really need to read this: Planning for Hub Transport Servers

Yes,just like mike said, please install Hub transport role with CAS role. More information share with you: Deployment Options for Hub Transport Servers http://technet.microsoft.com/en-us/library/bb124398(EXCHG.80).aspx Hope it helps. Xiu