Black Box LBG1000A, Manual, Page: 22 / 142

The purpose of IP multicast filtering is to optimize a switched network’s performance, so multicast packets will
only be forwarded to those ports containing multicast group hosts members and routers instead of flooding to all
ports in the subnet (VLAN).
Routing switches with IP multicast filtering/switching capability not only passively monitor IGMP Query and
Report messages, DVMRP Probe messages, PIM, and MOSPF Hello messages; they also actively send IGMP
Query messages to learn locations of multicast routers and member hosts in multicast groups within each VLAN.
Note, however, IGMP neither alters nor routes any IP multicast packets. Since IGMP is not concerned with the
delivery of IP multicast packets across subnetworks, an external IP multicast router is needed if IP multicast packets
have to be routed across different subnetworks.