System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP)

Multiple Directory Servers

Multiple LDAP servers can serve one DIT. For example, some subtrees of the DIT reside on other LDAP servers. In this case, an LDAP server may refer the LDAP client to a different server for the naming data it knows about but is not in its own database. If you plan such a DIT configuration, you should set the clients' profile attribute followReferrals to indicate to the LDAP naming service to follow server referrals to continue naming service lookups. However, it is best to have all naming data for a given domain reside on a single directory server, if at all possible.

Referrals can be useful if you want to have clients access read-only replicas most of the time and follow referrals to a read/write master server only when necessary. In this way, the master server does not get overloaded with requests that could be handled by replicas.