For every directory partition in a forest, there is an internal cross-reference object in the Partitions container:
cn=Partitions,cn=Configuration,dc= ForestRootDomain
When you create a new forest, the Active Directory Installation Wizard creates three directory partitions:
For each of these partitions, a cross-reference object and therefore a LDAP Referral is created automatically.
Because these cross-reference objects are located in the configuration directory partition, they are replicated to every Domain Controller in the forest, and thus every Domain Controller has knowledge of the name of every partition in the forest (as well as their superior and subordinate relationships to each other).
By virtue of this knowledge, any Domain Controller can generate LDAP Referrals to any other AD DOMAIN in the forest, as well as to the schema directory partition and configuration directory partition.