Unicast traffic refers to a one-to-one transmission from one point in the network to another. vSAN 6.6 and later uses unicast to simplify network design and deployment.
All ESXi hosts use the unicast traffic, and the vCenter Server becomes the source for the cluster membership. The vSAN nodes are automatically updated with the latest host membership list that vCenter provides. vSAN communicates using unicast for CMMDS updates.
Releases earlier than vSAN version 6.6 rely on multicast to enable the heartbeat and to exchange metadata between hosts in the cluster. If some hosts in your vSAN cluster are running earlier versions of the software, a multicast network is still required. The switch to unicast network from multicast provides better performance and network support. For more information on multicast, see Using Multicast in vSAN Network.