Users Guide

To accommodate these protocol enhancements, the IGMP version 3 packet structure is different from version 2. Queries (shown below in
query packet format) are still sent to the all-systems address 224.0.0.1, but reports (shown below in report packet format) are sent to all
the IGMP version 3 — capable multicast routers address 224.0.0.22.
Figure 11. IGMP version 3 Membership Query Packet Format
Figure 12. IGMP version 3 Membership Report Packet Format
Joining and Filtering Groups and Sources
The below illustration shows how multicast routers maintain the group and source information from unsolicited reports.
• The first unsolicited report from the host indicates that it wants to receive traffic for group 224.1.1.1.
• The host’s second report indicates that it is only interested in traffic from group 224.1.1.1, source 10.11.1.1. Include messages prevent
traffic from all other sources in the group from reaching the subnet, so before recording this request, the querier sends a group-and-
source query to verify that there are no hosts interested in any other sources. The multicast router must satisfy all hosts if they have
conflicting requests. For example, if another host on the subnet is interested in traffic from 10.11.1.3, the router cannot record the
include request. There are no other interested hosts, so the request is recorded. At this point, the multicast routing protocol prunes the
tree to all but the specified sources.
• The host’s third message indicates that it is only interested in traffic from sources 10.11.1.1 and 10.11.1.2. Because this request again
prevents all other sources from reaching the subnet, the router sends another group-and-source query so that it can satisfy all other
hosts. There are no other interested hosts, so the request is recorded.
Internet Group Management Protocol (IGMP)
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