Xerox NS-2260 - Manual (Page 15)

IGMP
Internet Group Management Protocol (IGMP) is used to dynamically register individual hosts in a multicast
group on a particular LAN. Hosts identify group memberships by sending IGMP messages to their local
multicast router. Under IGMP, routers listen to IGMP messages and periodically send out queries to
discover which groups are active or inactive on a particular subnet.
RFC 2236 defines the specification for IGMP Version 2. There are four types of IGMP messages:
_ Membership query
_ IGMP Version 1 membership report
_ IGMP Version 2 membership report
_ Leave group
Hosts send out IGMP membership reports corresponding to a particular multicast group to indicate that
they are interested in joining that group. The router periodically sends out an IGMP membership query to
verify that at least one host on the subnet is still interested in receiving traffic directed to that group. When
there is no reply to three consecutive IGMP membership queries, the router times out the group and stops
forwarding traffic directed toward that group.
With leave group message, the hosts can actively communicate to the local multicast router their intention
to leave the group. The router then sends out a group-specific query and determines whether there are any
remaining hosts interested in receiving the traffic. If there are no replies, the router times out the group and
stops forwarding the traffic.