Administrator Guide

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.
Figure 47. IGMP Membership Reports: Joining and Filtering
Leaving and Staying in Groups
The below illustration shows how multicast routers track and refreshes the state change in response to
group-and-specific and general queries.
• Host 1 sends a message indicating it is leaving group 224.1.1.1 and that the included filter for 10.11.1.1
and 10.11.1.2 are no longer necessary.
Internet Group Management Protocol (IGMP) 395