The presence of a single version 5 disk group in a vSAN version 6.6 cluster triggers the cluster to communicate permanently in unicast mode.
In an environment where a vSAN 6.6 cluster is already using an on-disk version 5 and a vSAN 6.5 node is added to the cluster, the following events occur:
- The vSAN 6.5 node forms its own network partition.
- The vSAN 6.5 node continues to communicate in multicast mode but is unable to communicate with vSAN 6.6 nodes as they use unicast mode.
A cluster summary warning appears on the on-disk format showing that one node is at an earlier version. You can upgrade the node to the latest version. You cannot upgrade disk format versions when a cluster is in a mixed mode.