Use referrals when entries are located on another server. The Directory Server implements two types of referrals depending on the requirement:

Referral on Update plugin
The Directory Server provides a Referral on Update plugin to create any referrals for update requests, such as add, delete, modify, or moddn operations, on read-only servers. For example, given two replicated directory servers where one server is a primary with read-write capabilities, and the other is a read-only server. You can configure a referral for any client update requests on the second directory server to point to the primary server. For example, if a client application sends an add request on the second directory server, the directory server responds with a referral that indicates any updates should be made on the primary server. All read requests on the read-only server process normally.
Smart referrals
The Directory Server supports smart referrals that map an entry or a specific branch of a directory information tree (DIT) to an LDAP URL. Any client requests (reads or writes) targeting at or below the branch of the DIT send a referral to the server designated in the LDAP URL.