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IGMP provides the final step in an IP multicast packet delivery service since it is only concerned with
the forwarding of multicast traffic from the local router to group members on directly attached
subnetworks. The Switch support IP Multicast Filtering by:
îš¾ Passively snooping on the IGMP Query and IGMP Report packets transferred between IP
Multicast Routers and IP Multicast host groups to learn IP Multicast group members, and
îš¾ Actively sending IGMP Query messages to solicit IP Multicast group members.
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).
The Switch 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 multi-cast 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 subnets, an external IP multicast router is needed if IP
multicast packets have to be routed across different subnets.
The IP multicast group perspective provides information associated with an IP multicast group. To
obtain an IP multicast group perspective:
1. From the L2 Switching DataBase screen, highlight IP Multicast Group Perspective and press the
Enter key. A screen similar to the following appears.
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