Repadmin /Add Command Fails with Error 8445 on Global Catalog Server (315562)

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Note: Microsoft recommends that you manually add replication links only under certain conditions -- for example, when a connection object cannot be created by the Knowledge Consistency Checker (KCC).

Each domain controller has only one read/write domain naming context (a naming context [NC] is equivalent to a directory partition). A global catalog server also has read-only domain naming contexts for all other domains in the forest. Although the repadmin /add command defaults to a read/write connection, the connection type must match the type of the destination partition. Therefore, when you add a replication link to a read-only partition on a global catalog server, you must specify the /readonly option.

The following example shows repadmin /add results under different conditions: The vm-child2-dc1.child2.root.com domain controller is in the child2.root.com child domain. When the domain controller is not a global catalog server and you add a replication link for another domain partition (in this case, the forest-root domain naming context dc=root,dc=com), this returns ERROR_DS_DRA_BAD_NC.

If the server is also a global catalog server, and /readonly is not specified, ERROR_DS_DRA_BAD_INSTANCE_TYPE is returned when you add the replication link, as follows:
The operation succeeds if /readonly is specified: