Mark has been reading this book and said look at the tip on page 34, I have now lost all respect for this book.
So page 34 says:
To reduce network traffic between Exchange Servers, Domain Controllers and Global Catalog Servers you’ll typically want to deploy mailbox, cas, hub & um roles on a computer acting as a Global Catalog. This is not a requirement best practice for most exchange implementations. Edge severs are not members of the ad forest & are not configured ad gcs
Humm, so this MICROSOFT article (http://technet.microsoft.com/en-us/library/aa996719.aspx) says:
Installing Exchange 2007 on Directory Servers
For security and performance reasons, we recommend that you install Exchange 2007 only on member servers, and not on Active Directory directory servers. While installing Exchange 2007 on a directory server is supported, it is strongly discouraged. However, you cannot run DCPromo on a computer running Exchange 2007. Once Exchange 2007 is installed, changing its role from a member server to a directory server, or vice versa, is not supported.
So who is right? Me thinks Microsoft