Reference Guide

IGMP Version 3
Conceptually, IGMP version 3 behaves the same as version 2. However, there are differences.
• Version 3 adds the ability to filter by multicast source, which helps multicast routing protocols avoid forwarding
traffic to subnets where there are no interested receivers.
• 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
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