>= (greater or equal) and <= (lower or equal) operators in search filter

["Alexandre Garel" <garel.alexandre@agora.msa.fr>]

Hie,
according to RFC 2254, one can use >= and <= operators in a search filter.
When I try it on an attribute like uid and cn, with openldap ver 2.1.22 
it does not return an error but a 0 results and in logs I can see  
filter="(?=undefined)".
Is that because
- openldap ver 2.1.22 does not implement that part of RFC 2254. In this 
case, does newer version do this (and which one) ? (although 
http://www.openldap.org/lists/openldap-software/200208/msg00598.html 
seems to tell it's already implemented).
- uid and cn have no ordering  ? In this case, what shall I add to the 
schema to make use of these operators on an attribute ?
Thanks