Exchange 2010 Backup

Hi all

We have an Exchange 2010 environment with 3 nodes in the DAG group.  2 of the nodes are local and the third is at our DR site.  Normally we are running on one of the 2 local nodes.  We had an issue where we had to shut down the DR node and now my DAG backups fail with "1: (26) client/server handshaking failed ".  Do all 3 nodes have to be up to backup even if they are in passive mode?  In the Exchange backup policy under "Preferred servers" I don't even list the third server since I don't want to back him up anyway.

Thanks

Enio