What does the error "unsupported service type ExternalName" mean in Istio Primary Remote multi cluster?

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The error "unsupported service type ExternalName" in Istio Primary Remote multi-cluster means that Istio does not support the use of the ExternalName service type in a multi-cluster setup. ExternalName is used to map a Kubernetes Service directly to an external DNS name. This is not compatible with Istio's multi-cluster setup as it requires Istio to be aware of the IP addresses of all services in all clusters, which is not possible with ExternalName. Therefore, you may need to use a different service type, such as ClusterIP, to enable Istio Primary Remote multi-cluster.