DAG setup

Hi Team,

I had doubt in create the DAG setup in my environment.

I have 4 servers in my environment.one is "Domain Controller" name is "dagserver.dag.com"

The remaining three servers i need to install "Active Directory". For creating the DAG setup How can i install "Active Directory" in  the remaining three server ?

which option i want to choose while installing the "Active Directory"?

here there are two options in "Active Directory configuration"

1.Existing forest

            -->Add a domain controller to an existing domain

            -->create a new domain in existing forest

2.Create a new domain in new forest.

Which option i can choose(1 or 2) for creating the DAG setup?

can anybody help me?

regards,

Gousemeera