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It is completely unaware of the data (protocol) being carried in the data portion of the frame, such as an IPv4 packet, an ARP message or an IPv6 ND packet. Switch Fundamentals -A Layer 2 Ethernet switch uses Layer 2 MAC addresses to make forwarding decisions. As with the unicast and broadcast addresses, the multicast IP address requires a corresponding multicast MAC address.

The source will always be a unicast address. Because multicast addresses represent a group of addresses (sometimes called a host group) they can only be used as the destination of a packet. It is not forwarded by a router, unless the router is configured to route multicast packets. It is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping. There are other reserved multicast destination MAC addresses for when the encapsulated data is not IP, such as Spanning Tree Protocol (STP) There is a destination MAC address of 01-00-5E when the encapsulated data is an IPv4 multicast packet and a destination MAC address of 33-33 when the encapsulated data is an IPv6 multicast packet Multicast MAC Address An Ethernet multicast frame is received and processed by a group of devices that belong to the same multicast group.