Users Guide
• To enable ltering, routers must keep track of more state information, that is, the list of sources that must be ltered. An
additional query type, the Group-and-Source-Specic Query, keeps track of state changes, while the Group-Specic and General
queries still refresh the existing state.
• Reporting is more ecient 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 dierent 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 43. IGMP Version 3 Packet Structure
Figure 44. 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 rst unsolicited report from the host indicates that it wants to receive trac for group 224.1.1.1.
2. The host’s second report indicates that it is only interested in trac from group 224.1.1.1, source 10.11.1.1. Include messages
prevents trac 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 conicting requests. For example, if another host on the subnet is interested in trac 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 specied sources.
Internet Group Management Protocol (IGMP)
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