man ldapsearch
...
-S attribute
Sort the entries returned based on attribute. The default is not to sort entries returned. If attribute is a zero-length string (""), the entries are sorted by the components of their Distingished Name. See ldap_sort(3) for more details. Note that ldapsearch normally prints out entries as it receives them. The use of the -S option defeats this behavior, causing all entries to be retrieved, then sorted, then printed.
ciao