Dell PowerEdge FX2/FX2s - Manual (Page 85)

• 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 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.
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.
IGMP Overview 85