3 Any remaining hosts respond to the query according to the delay timer mechanism (refer to Adjusting Query and Response Timers). If
no hosts respond (because there are none remaining in the group), the querier waits a specied period and sends another query. If it
still receives no response, the querier removes the group from the list associated with forwarding port and stops forwarding trac for
that group to the subnet.
IGMP Version 3
Conceptually, IGMP version 3 behaves the same as version 2. However, there are dierences.
• Version 3 adds the ability to lter by multicast source, which helps multicast routing protocols avoid forwarding trac to subnets where
there are no interested receivers.
• 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 35. IGMP Version 3 Packet Structure
Internet Group Management Protocol (IGMP)
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