change ISTG role immediately using command.

You want to move the ISTG for a site to another domain controller. This happens automatically if you take the current ISTG offline, but you may want to transfer the role to a server that is more optimal in your environment.

11.26.2 Solution
11.26.2.1 Using a graphical user interface
Open ADSI Edit.

Connect to the CNC if it is not already displayed in the left pane.

In the left pane, browse the Configuration NC Sites.

Click on the site you want to transfer the ISTG for.

In the right pane, double-click CN=NTDS Site Settings.

Modify the interSiteTopologyGenerator attribute to include the NTDS Settings object of the domain controller you want to transfer the ISTG role to.

Click OK.

11.26.2.2 Using VBScript
' This code forces a new ISTG in a site.
' ------ SCRIPT CONFIGURATION ------
' Name of site to transfer ISTG in
strSiteName = "<SiteName>" ' e.g. Raleigh
' Site the new ISTG server is in
strNewISTGSite = "<ISTGSiteName>" ' e.g. Raleigh
' Common name of server object for new ISTG
strNewISTGName = "<DomainControllerName>" ' e.g. dc01
' ------ END CONFIGURATION ---------

set objRootDSE = GetObject("LDAP://RootDSE")
set objSiteSettings = GetObject("LDAP://cn=NTDS Site Settings,cn=" & _
strSiteName & ",cn=sites," & _
objRootDSE.Get("ConfigurationNamingContext"))
strCurrentISTG = objSiteSettings.Get("interSiteTopologyGenerator")

objSiteSettings.Put "interSiteTopologyGenerator", _
"cn=NTDS Settings,cn=" & strNewISTGName & _
",cn=servers,cn=" & strNewISTGSite & ",cn=sites," & _
objRootDSE.Get("ConfigurationNamingContext")
objSiteSettings.SetInfo
WScript.Echo "ISTG for " & strSiteName & " changed from:"
WScript.Echo " " & strCurrentISTG
WScript.Echo "To"
WScript.Echo " " & objSiteSettings.Get("interSiteTopologyGenerator")

You can follow along here.
https://www.peppercrew.nl/index.php/2011/05/moving-the-istg-role-to-another-domain-controller/

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