# Multicast MAC with IGMP

Internet Group Management Protocol (IGMP) can be used in combination with multicast MAC addresses to avoid flooding with switches that do not support statically defined destinations for multicast.

In this mode, switches are configured to send multicast traffic only to the ports from which they have received IGMP Host Membership Report messages corresponding to the MAC address in question. Multicast with IGMP must be selected as the mode for the cluster, and IGMP snooping enabled on the switch. For the IGMP messaging, a common multicast IP address for the cluster nodes should be specified. The multicast MAC address is then computed automatically based on it. Do note, however, that the CVIs are still identified solely by the common unicast IP address; the multicast IP address is only used as the source address for the IGMP messages sent to the switch.

Note: Some routers that use router redundancy protocols such as HSRP or VRRP listen to all multicast traffic in addition to the routing-related traffic. Thus, multicast packets are rerouted to the network. To prevent the rerouting, you can either configure the router to send this traffic only to the cluster ports or define the router’s access control list (ACL) to drop all incoming packets with the cluster’s multicast MAC.