vSphere Replication server registration might take a long time depending on the number of hosts in the vCenter Server inventory.

If the vCenter Server inventory contains a few hundred or more hosts, the Register VR Server task takes more than a few minutes to complete.

vSphere Replication updates each host's SSL thumbprint registry. The vCenter Server Events pane displays Host is configured for vSphere Replication for each host as the vSphere Replication server registration task progresses.

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Wait for the registration task to complete.

After it finishes, you can use vSphere Replication for incoming replication traffic.

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Alternatively, edit /opt/vmware/hms/conf/hms-configuration.xml and change hms-config-host-at-hbr-threadpool-size parameter to a higher value to enable parallel processing of more hosts at a time and restart the vSphere Replication management server /etc/init.d/hms restart