The availability of a single version 5 disk group in vSAN version 6.6 disk group triggers the cluster to communicate permanently in the unicast mode.
- All cluster hosts are running vSAN version 6.5 or lower.
- All disk groups are using on-disk version 3 or earlier.
- A non-vSAN 6.6 host such as vSAN 6.2 or vSAN 6.5 is added to the cluster.
For example, if a host running vSAN 6.5 or earlier is added to an existing vSAN 6.6 cluster, the cluster reverts to multicast mode and includes the 6.5 host as a valid node. To avoid this behavior, use the latest version for both ESXi hosts and on-disk format. To ensure that vSAN cluster continues communicating in unicast mode and does not revert to multicast, upgrade the disk groups on the vSAN 6.6 hosts to on-disk version 5.0.