3Com 4007, Implementation Manual, Page: 433 / 684

How IGMP Supports IP Multicast 433
Join Message
Rather than wait for a query, a host can also send an IGMP report on its
own initiative to inform the querier that it wants to begin receiving a
transmission for a specific group (perhaps by clicking a
Go or Start button
on the client interface). This is called a join message. The benefit is faster
transmission linkages, especially if the host is the first group member on
the subnetwork.
Leave-Group Messages
Leave-group messages are a type of host message defined in IGMP
version 2. If a host wants to leave an IP multicast group, it issues a
leave-group message addressed to 224.0.0.2, the
all routers in this
subnetwork Class D address. Upon receiving such a message, the querier
determines whether that host is the last group member on the
subnetwork by issuing a
group-specific query .
Leave-group messages lower leave latency — that is, the time between
when the last group member on a given subnetwork sends a report and
when a router stops forwarding traffic for that group onto the
subnetwork. This process conserves bandwidth. The alternative is for the
router to wait for at least two queries to go unanswered before pruning
that subnetwork from the delivery tree.
Role of IGMP in IP
Multicast Filtering
To further refine the IP multicast delivery process and maximize
bandwidth efficiency, a Layer 3 module filters IP multicast packets on
appropriate ports using a process called
IGMP snooping
. Both bridged
interfaces and routed interfaces record which ports receive host IGMP
reports and then set their filters accordingly so that IP multicast traffic for
particular groups is not forwarded on ports or VLANs that do not require
it.