Multicast is a network communication technique that sends information packets to a group of destinations over an IP network.

Releases earlier than vSAN version 6.6 support IP multicast and used IP multicast communication as a discovery protocol to identify the nodes trying to join a vSAN cluster. Releases earlier than vSAN version 6.6 depend on IP multicast communication while joining and leaving the cluster groups and during other intra-cluster communication operations. Ensure that you enable and configure the IP multicast in the IP network segments to carry the vSAN traffic service.

An IP multicast address is called a Multicast Group (MG). IP multicast sends source packets to multiple receivers as a group transmission. IP multicast relies on communication protocols that hosts, clients, and network devices use to participate in multicast-based communications. Communication protocols such as Internet Group Management Protocol (IGMP) and Protocol Independent Multicast (PIM) are the main components and dependencies for the use of IP multicast communications.

While creating a vSAN cluster, a default multicast address is assigned to each vSAN cluster. The vSAN traffic service automatically assigns the default multicast address settings to each host. This multicast address sends frames to a default multicast group and multicast group agent.

When multiple vSAN clusters reside on the same Layer 2 network, VMware recommends changing the default multicast address within the additional vSAN clusters. This prevents multiple clusters from receiving all multicast streams. See VMware KB 2075451 for more information about changing the default vSAN multicast address.

Internet Group Management Protocol

You can use Internet Group Management Protocol (IGMP) to add receivers to the IP Multicast group membership within the Layer 2 domains.

IGMP allows receivers to send requests to the multicast groups they want to join. Becoming a member of a multicast group allows routers to forward traffic for the multicast groups on the Layer 3 segment where the receiver is connected to switch port.

You can use IGMP snooping to limit the physical switch ports participating in the multicast group to only vSAN VMkernel port uplinks. IGMP snooping is configured with an IGMP snooping querier. The need to configure an IGMP snooping querier to support IGMP snooping varies by switch vendor. Consult your specific switch vendor for IGMP snooping configuration.

vSAN supports both IGMP version 2 and IGMP version 3. When you perform the vSAN deployment across Layer 3 network segments, you can configure a Layer 3 capable device such as a router or a switch with a connection and access to the same Layer 3 network segments.

All VMkernel ports on the vSAN network subscribe to a multicast group using IGMP to avoid multicast flooding all network ports.

Note: You can deactivate IGMP snooping if vSAN is on a non-routed or trunked VLAN that you can extend to the vSAN ports of all the hosts in the cluster.

Protocol Independent Multicast

Protocol Independent Multicast (PIM) consists of Layer 3 multicast routing protocols.

It provides different communication techniques for IP multicast traffic to reach receivers that are in different Layer 3 segments from the multicast groups sources. For earlier vSAN version 6.6 cluster, you must use PIM to enable the multicast traffic to flow across different subnets. Consult your network vendor for the implementation of PIM.