Users Guide

• To enable filtering, routers must keep track of more state information, that is, the list of sources that must be filtered. An additional
query type, the Group-and-Source-Specific Query, keeps track of state changes, while the Group-Specific and General queries still
refresh the existing state.
• Reporting is more efficient and robust: hosts do not suppress query responses (non-suppression helps track state and enables the
immediate-leave and IGMP snooping features), state-change reports are retransmitted to insure delivery, and a single membership
report bundles multiple statements from a single host, rather than sending an individual packet for each statement.
The version 3 packet structure is different from version 2 to accommodate these protocol enhancements. Queries are still sent to the all-
systems address 224.0.0.1, as shown in the following illustration, but reports are sent to the all IGMP version 3-capable multicast routers
address 244.0.0.22, as shown in the second illustration.
Figure 44. IGMP Version 3 Packet Structure
Figure 45. IGMP Version 3–Capable Multicast Routers Address Structure
Joining and Filtering Groups and Sources
The following illustration shows how multicast routers maintain the group and source information from unsolicited reports.
1. The first unsolicited report from the host indicates that it wants to receive traffic for group 224.1.1.1.
2. 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 prevents
traffic from all other sources in the group from reaching the subnet. 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.
Internet Group Management Protocol (IGMP)
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