Before adding new hosts to a stretched cluster, all current hosts must be connected. If a current host is disconnected, the configuration of the new host is incomplete.
After you add a new host to a stretched cluster in which some hosts are disconnected, on the Summary tab for the cluster the Configuration Status for Virtual SAN appears as Unicast agent unset on host.
When a new host joins a stretched cluster, Virtual SAN must update the configuration on all hosts in the cluster. If one or more hosts are disconnected from the vCenter Server, the update fails. The new host successfully joins the cluster, but its configuration is incomplete.
Verify all hosts are connected to vCenter Server, and click on the link provided in the Configuration Status message to update the configuration of the new host.
If you cannot rejoin the disconnected host, remove the disconnected host from the cluster, and click on the link provided in the Configuration Status message to update the configuration of the new host.