4613 Known Issues in NDB Cluster Replication acts as the replication source for Cluster 2, Cluster 2 acts as the source for Cluster 3, and Cluster 3 acts as the source for Cluster 1, thus completing the circle. Each NDB Cluster has two SQL nodes, with SQL nodes A and B belonging to Cluster 1, SQL nodes C and D belonging to Cluster 2, and SQL nodes E and F belonging to Cluster 3. Circular replication using these clusters is supported as long as the following conditions are met: This type of circular replication setup is shown in the following diagram: In this scenario, SQL node A in Cluster 1 replicates to SQL node C in Cluster 2; SQL node C replicates to SQL node E in Cluster 3; SQL node E replicates to SQL node A. In other words, the replication line (indicated by the curved arrows in the diagram) directly connects all SQL nodes used as sources and replicas. It should also be possible to set up circular replication in which not all source SQL nodes are also replicas, as shown here: •All SQL nodes acting as sources and replicas are started with the  log_slave_updates  system variable enabled. •The SQL nodes on all source and replica clusters are the same. Figure 23.32 NDB Cluster Circular Replication With All Sources As Replicas