ol plane if its cluster ID
is specified with a <code>topology.istio.io/controlPlaneClusters</code> annotation on the system namespace of the
remote cluster. This annotation must be added to a remote cluster BEFORE upgrading the corresponding
control plane on an external or primary cluster.</p><p>Refer to the <a href=/v1.19/docs/setup/install/external-controlplane/#register-the-new-cluster>external control plane</a>
and <a href=/v1.19/docs/setup/install/multicluster/primary-remote/#attach-cluster2-as-a-remot