The root DSE is a special entry that contains information related to the current server instance, such as a list of supported suffixes, available authentication mechanisms, and so forth. You can search this entry by supplying a search base of “”. You must also specify a search scope of base and a filter of "(objectclass=*)".
For example:
ldapsearch -h myServer -p 5201 -D cn=admin,cn=Administrators,cn=config -w - -b "" -s base "(objectclass=*)"