IP multicast is a method of sending Internet Protocol (IP) datagrams to a group of interested receivers in a single transmission. It is often employed for streaming media applications on the Internet and private networks. Since traffic is sent from the source once in total, instead of once per recipient, throughput can be saved.
Each host (and in fact each application on the host) that wants to be a receiving member of a multicast group (i.e. receive data corresponding to a particular multicast address) must use the Internet Group Management Protocol (IGMP) to join. A detailed article on how nodes join and leave multicast groups can be found here. Below is a packet capture with details of the joining process.
The first packet, a membership query, is sent when a member wants to know about the other members of a multicast group. The membership reports are sent by the various members, announcing that they belong to the group. These membership reports are also how a member joins the group in the first place. The leave group message is the opposite of a membership report, and indicates that the member is no longer part of the multicast group.
IGMP snooping is the process of listening to IGMP network traffic. The feature allows a network switch to listen in on the IGMP conversation between hosts and routers. By listening to these conversations the switch maintains a map of which links need which IP multicast streams. Multicasts are filtered from the links which do not need them and thus controls which ports receive specific multicast traffic.
MX Security Appliances will forward IGMP traffic for a single broadcast domain. It does not forward multicast traffic upstream, between VLANs, or over a VPN.
MS Switches can forward IGMP traffic, but will run IGMP snooping by default. This prevents the switch from sending multicast traffic to hosts who are not yet joined with the proper multicast group. If IGMP snooping needs to be disabled, please contact Cisco Meraki support.
PIM-SM support is now in public beta. Please contact Cisco Meraki support to enable this feature. For more information, see MS Multicast Routing Overview.
An IGMP querier is required for IGMP snooping to operate per RFC4606. IGMP querier is now available in public beta. Please contact Cisco Meraki support to enable this feature.
To configure an IGMP querier:
The MR Access Points will forward multicast and IGMP traffic, but does not itself participate in the process. This means that an MR Access Point will not prevent clients from joining a mutlicast group, but it will not itself be a destination. In addition, multicast-to-unicast is enabled on all MR Access Points by default. More information on this behavior can be found here.