Administrator Guide

a specified 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 traffic for that group to the
subnet.
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 the 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 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 45. IGMP version 3 Membership Query Packet Format
Figure 46. IGMP version 3 Membership Report Packet Format
Internet Group Management Protocol (IGMP) 394