When synchronous replication fails immediately between two clusters, it is often due to blocked communication across the required ports (2009 and 2020). These must be open manually if the clusters are in different subnets or if network policies block traffic.
From theNutanix Enterprise Cloud Administration (ECA)course materials:
"Replication communication relies on specific ports, which must be allowed through the firewall. If the clusters are in different subnets or if there are external firewalls, these ports must be explicitly opened."
"The modify_firewall command is used to open or close ports on cluster nodes. For replication and remote site communication, eth1 is typically used for external replication traffic." Since eth1 is typically used for external connectivity and replication traffic, opening ports on this interface using modify_firewall resolves the communication block.