Identify the ISTG's
To locate the ISTG role holders for all sites
Click Start, click Run, type Ldp, and then click OK.
On the Connection menu, click Connect.
In the Connect dialog box, leave the Server box empty.
In Port, type 389, and then click OK.
On the Connection menu, click Bind.
In the Bind dialog box, provide Enterprise Admins credentials. Click Domain if it is not already selected.
In Domain, type the name of the forest root domain, and then click OK.
On the Browse menu, click Search.
In Base dn, type:CN=Sites,CN=Configuration,DC= Forest_Root_Domain
In Filter, type: (CN=NTDS Site Settings)
For Scope, click Subtree.
Click Options, and in the Attributes box, scroll to the end of the list, type: ;interSiteTopologyGenerator
and then click OK.
In the Search dialog box, click Run.
Review the interSiteTopologyGenerator entries in the output, and make a note of the domain controller names.