Superior knowledge information may be specified using the referral directive. The value is a list of URIs referring to superior directory services. For servers without immediate superiors, such as for a.example.net in the example above, the server can be configured to use a directory service with global knowledge, such as the OpenLDAP Root Service (https://www.openldap.org/faq/index.cgi?file=393).
However, as a.example.net is the immediate superior to b.example.net, b.example.net would be configured as follows:
The server uses this information to generate referrals for operations acting upon entries not within or subordinate to any of the naming contexts held by the server.
For those familiar with X.500, this use of the ref attribute is similar to an X.500 knowledge reference held in a Supr DSE.