The Ethernet address space is divided into 3 categories: Unicast, Multicast and Broadcast. Unicast messages are intended for a single recipient, multicast messages for more than one recipient, and broadcast messages are intended for all recipients on the network. Unicast messages are the most common variety in an Ethernet network since most communication is point-to-point. While Broadcast messages are used chiefly for inter-equipment communication, multicast functionality can be quite useful in commercial services. As an example, in an IP-TV network, a given channel is multicast to all the subscribers who are currently viewing the channel. This can also be emulated by multiple Unicast messages, but that would be bandwidth inefficient since multiple copies of the same message would be sent on the same links.
All multicast-capable switches have a multicast table as shown:
 Multicast Table in a Switch
Since Multicast MAC addresses are never the source of information, they cannot be learned the way Unicast MAC addresses are learned. Hence, the multicast tables are filled by the switches in 2 ways:
- IGMP snooping: Internet Group management protocol (IGMP), a Layer-3 multicasting protocol, is used by IP hosts to report their host group memberships to any immediately neighboring multicast routers. When a host wants to join an IP multicast group, it sends an IGMP join message. For example, in an IP-TV network, a Set-top box would send an IGMP message to the IPTV server Router whenever it wants to join the group of viewers watching a particular channel. The intermediate Ethernet switches can ‘snoop’ on these IGMP messages and form a corresponding Ethernet multicast group. The source nodes originating the IGMP requests can be automatically joined to the Ethernet multicast groups by the intermediate Ethernet switches.
- GARP Multicast Registration GMRP: GARP Multicast Registration Protocol (GMRP, defined by the IEEE 802.1P) provides a constrained multicast flooding facility similar to IGMP snooping. GMRP provides a mechanism that allows Ethernet switches and client nodes to dynamically register group membership information with the switches attached to the same network segment and for that information to be disseminated across all Ethernet switches in the network. When a host wants to join an IP multicast group, it sends an IGMP join message, which spawns a GMRP join message. Upon receipt of the GMRP join message, the switch adds the port through which the join message was received to the appropriate multicast group. The switch propagates the GMRP join message to all other hosts in the VLAN, one of which is typically the multicast source.
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