currently our environment has 3 DCs .
primary DC ,
1st PDC is windows server 2012 standard ( not R2 ),
2nd DCs windows server 2012 standard ( not R2 ) and
3rd Dc is windows server 2008 R2 standard.
we are planning to upgrade to windows server 2016 DC and for the first step I was trying to promote it to a domain controller but not successful .
when I tried to run the prerequisite check in dcpromo promoting it as a domain controller from PDC the error said not able to find the PDC.
>>> verification of outbound replication failed unable to locate replication source domain controller <<<
But when I tried the prerequisite check with my 3rd Dc ( windows server 2008 )the prerequisite was passed.
but I was not able to promote it from there because the 3rd 2008 server was not PDC.