vSAN prerequisites advise you to always enable IP multicast on the physical switches and routers, which handle vSAN traffic along with the layer 2 path and the layer 3 path (optionally). It is used mostly for intra-cluster communication for heartbeats and exchange of metadata between the ESXi hosts participating in the cluster. Multicast traffic can be designed for named port groups, by using IGMP (v3) snooping. VMware best practices advise you to not implement multicast flooding across all ports.

Multicast communication is not configured on most switches. There is no IGMP snooper setup so traffic will not pass. Design an IGMP snooper so traffic can pass, or explicitly disable IGMP snooping on the port to be sure that multicast traffic is passed.

VMware best practices tell you to enable multicast only on the segments or ports that are being used for Virtual SAN.