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.

vSAN version 6.6 clusters automatically revert to multicast communication in the following situations:
  • 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.

Note: Avoid having a mixed mode cluster where vSAN version 6.5 or earlier are available in the same cluster along with vSAN version 6.6 or later.