IGMPv1 defines the group membership query and report processes. IGMPv2 extends IGMPv1 by adding the querier election and member leave mechanisms. IGMPv3 extends IGMPv2 by allowing hosts to specify the multicast sources they do or do not want to receive data from.
All IGMP versions support the any-source multicast (ASM) model. IGMPv3 can be directly applied to the source-specific multicast (SSM) model. IGMPv1 and IGMPv2, however, can be applied to the SSM model only when IGMP SSM mapping is configured. For details about the ASM and SSM models, see Multicast Service Models.
Table 1 compares the three IGMP versions.
Item |
IGMPv1 |
IGMPv2 |
IGMPv3 |
|---|---|---|---|
Querier election |
Through Protocol Independent Multicast (PIM) |
Through competition among multicast routers on a local network segment |
Through competition among multicast routers on a local network segment |
General Query message |
Supported |
Supported |
Supported |
Report message |
Supported |
Supported |
Supported |
Group-Specific Query message |
Not supported |
Supported |
Supported |
Leave message |
Not supported |
Supported |
No specific Leave message is defined. Instead, group members send a specified type of Report message to notify multicast routers that they are leaving a group. |
Group-and-Source-Specific Query message |
Not supported |
Not supported |
Supported |
Specifying multicast source in Report messages |
Not supported |
Not supported |
Supported |
Protocol message versions supported |
IGMPv1 |
IGMPv1, IGMPv2 |
IGMPv1, IGMPv2, IGMPv3 |
ASM model |
Supported |
Supported |
Supported |
SSM model |
IGMP SSM mapping required |
IGMP SSM mapping required |
Supported |
IGMP versions are backward compatible. This means that a multicast router running a later IGMP version can identify IGMP messages sent from hosts running an earlier IGMP version, despite the format of IGMP messages in different versions being different. For example, a multicast router running IGMPv2 can correctly process Report messages from IGMPv1 hosts, and a multicast router running IGMPv3 can correctly process Report messages from IGMPv1 and IGMPv2 hosts. When a multicast router receives an IGMP Report messages from a host running an earlier IGMP version, the router automatically downgrades the version of the corresponding multicast group so it can operate in the same version as the host.
For example, when a multicast router running IGMPv2 or IGMPv3 receives a Report message from an IGMPv1 host, the router downgrades the version of the corresponding multicast group to IGMPv1. Then, the router ignores the IGMPv2 Leave messages for this multicast group. In addition, when a multicast router running IGMPv3 receives a Report message from an IGMPv2 host, the router downgrades the version of the corresponding multicast group to IGMPv2. Then, the router ignores the multicast source list in IGMPv3 BLOCK messages, TO_IN messages, and TO_EX messages, which prohibits the hosts from selecting multicast sources. If a multicast router is manually upgraded to a later version, a multicast group will still work in the earlier IGMP version until all hosts running this version have left the multicast group.