setting up replica on domain controller

What?

If you have one domain controller right now, are you looking to add a second one? If so, yes, this is pretty easy to do and very common.

Exchange's directory service piece is really just active directory, so that should be covered by having a seconds DC.

If you are looking to have a duplicate copy of the mailboxes on your main exchange server, then you are looking for some third party software, such as Double-take, to keep replicating the changes over.

What exactly do you want to achieve?

You can't replicate Exchange mailboxes to another machine natively.
If you do want to replicate Exchange using a third party tool, then you have to use a dedicated machine as the replication tool replicates everything and become the other machine in the event of a failure. You cannot have the replica machine doing something else in the meantime.

Simon.

Simon,

Thanks for your responcse.

This is what I wanted to find out if I need another machine for replica only? I have only 50 users and they will not grow. I have one machine which is the Opration master for the domain and it is not doing any other services. I was thinking of using this machine for exchantge's replica.

What kind of tools do I need to setup for backup/replica/mirror/sync which allow to bring Exchange server right away in the event of any failure? like hot swapable... can it be done?