It is a good idea to protect the bandwidth available within a collision domain by blocking off multicast traffic that is not addressed to a station in that domain.
It is more difficult to block off multicast traffic than to block off unicast traffic.
IP multicast addresses are mapped into Ethernet or FDDI MAC ddresses. The first 25 bits of the MAC address hold a fixed-bit pattern. The last 23 bits of the MAC address are copied from the last 23 bits of the multicast IP address.
IGMP snooping is one method used to discover which parts of a LAN need to receive multicasts sent to a particular group.
To perform IGMP snooping, bridges eavesdrop on the IGMP messages that stations use to join and leave multicast groups.
IGMP snooping is CPU-intensive. ...