Istio Multicluster is a feature of Istio–the basis of Red Hat OpenShift Service Mesh–that allows for the extension of the service mesh across multiple Kubernetes or Red Hat OpenShift clusters. The primary goal of this feature is to enable control of services deployed across multiple clusters with a single control plane. The main requirement for Istio multicluster to work is that the pods in the mesh and the Istio control plane can talk to each other.
This implies that pods need to be able to open connections between clusters.
Source: openshift.com