Openldap Replication don't run after configuring
The directives updatedn and updateref do not make sense at the provider (master).

You probably also want type=refreshAndPersist for continous replication. Setting interval= to one second would not work well with many entries in the DB.

Make sure that attributes entryUUID and entryCSN are indexed at the provider.

Also note that OpenLDAP 2.4.21 is really ancient release. During the last 9 years many things were fixed in OpenLDAP, especially related to replication.

I'd recommend to use the RPMs built by LDAP Toolbox project. They provide YUM repos also for RHEL/CentOS 6.

See also: OpenLDAP Admin Guide -- Replication