You must assign an access node to a Defender security server in order for machines assigned to that access node to know where to send one-time passwords for authentication.
To assign the access node to the security server
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Open Active Directory Users and Computers. 
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Under the Defender node, open Access Nodes. 
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Double-click the access node that you created previously. 
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On the Access Node tab, click Assign. 
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Select your Defender security server from the list and click OK. 
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Click OK to save your changes to the access node. 
