User's Manual

User’s Manual of IGS-4215 Series
176
IGMP packets enable multicast routers to keep track of the membership of multicast groups, on their respective sub networks.
The following outlines what is communicated between a multicast router and a multicast group member using IGMP.
A host sends an IGMP “report” to join a group
A host will never send a report when it wants to leave a group (for version 1).
A host will send a “leave” report when it wants to leave a group (for version 2).
Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group members
exist on their sub networks. If there is no response from a particular group, the router assumes that there are no group
members on the network.
The Time-to-Live (TTL) field of query messages is set to 1 so that the queries will not be forwarded to other sub networks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast queried for each LAN, an explicit leave
message, and query messages that are specific to a given group.
The states a computer will go through to join or to leave a multicast group are shown below:
Figure 4-7-6: IGMP State Transitions