we have our first primary server set up and patched to 11.2.2

external MSSQL database 2008R2

When I try to install a second primary server to the zone, I get the error :

"Unable to contact the database server. The database address specified by the parent primary server can not be resolved by this server. This usually indicates either a DNS resolution problem or that the database server could not be reached from this server. Try specifying your first primary server as the parent of this server."

as far as I can tell, DNS is correct for the database server. ZDM.xml seems to be pointing to the right database.

Any help on this please? I'm totally stuck!