adding an index to an attribute:

addAttribute.txt:

dn: cn=catalogs
changetype: modify
add: orclindexedattribute
orclindexedattribute: departmentnumber

ldapmodify -h localhost -p 389 -D "cn=orcladmin" -f addAttribute.txt

ps: there should not be any objects having this attribute. meaning: put the index first, then create/import objects.