Here are some notes I have based on the setup I've done:

#####  REPLICATION ON Master #####

[root@ldap01 ~]# more mod_syncprov.ldif
dn: cn=module,cn=config
objectClass: olcModuleList
cn: module
olcModulePath: /usr/lib64/openldap
olcModuleLoad: syncprov.la

[root@ldap01 ~]# ldapadd -Y EXTERNAL -H ldapi:/// -f mod_syncprov.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
adding new entry "cn=module,cn=config"

[root@ldap01 ~]# more syncprov.ldif
dn: olcOverlay=syncprov,olcDatabase={2}hdb,cn=config
objectClass: olcOverlayConfig
objectClass: olcSyncProvConfig
olcOverlay: syncprov
olcSpSessionLog: 100
[root@ldap01 ~]#

[root@ldap01 ~]# ldapadd -Y EXTERNAL -H ldapi:/// -f syncprov.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
adding new entry "olcOverlay=syncprov,olcDatabase={2}hdb,cn=config"

[root@ldap01 ~]#

#####  REPLICATION ON Slave #####

[root@ldap02 ~]# more syncrepl.ldif
dn: olcDatabase={2}hdb,cn=config
changetype: modify
add: olcSyncRepl
olcSyncRepl: rid=001
  provider=ldap://172.19.33.42:389/
  bindmethod=simple
  binddn="cn=admin,dc=ZZZ,dc=ZZZ"
  credentials=password
  searchbase="dc=ZZZ,dc=ZZZ"
  scope=sub
  schemachecking=on
  type=refreshAndPersist
  retry="30 5 300 3"
  interval=00:00:05:00

[root@ldap02 ~]# ldapadd -Y EXTERNAL -H ldapi:/// -f syncrepl.ldif
SASL/EXTERNAL authentication started
SASL username: gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth
SASL SSF: 0
modifying entry "olcDatabase={2}hdb,cn=config"

[root@ldap02 ~]#

# Testing LDAP
ldapsearch -x -b 'uid=testaccount,ou=People,dc=ZZZ,dc=ZZZ'

Both LDAP hosts work in the sense that I can authenticate to them.  But replication is not working.  

Please help.

TIA