Users 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 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 11. IGMP version 3 Membership Query Packet Format

Figure 12. IGMP version 3 Membership Report Packet Format

Internet Group Management Protocol (IGMP)