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ROS™ v3.5 196 RS400
Multicast
Router
P1
C1 C2 C3 C4
M1
M2
Membership Query
M1 Membership Report
Membership Query
M2 Membership Report
Figure 135: IGMP Operation Example 1
In this example the general membership query sent to the C1-C2 segment is answered by a
membership report indicating the desire to subscribe to a stream M2. The router will forward the
M2 stream onto the C1-C2 segment. In a similar fashion the router discovers that it must
forward M1 onto segment C3-C4.
Note: Membership reports are also referred to as “joins”.
A consumer may join any number of multicast groups, issuing a membership report for each
group. Hosts on the segment note membership reports from other hosts and will suppress their
own reports accordingly. In this way the IGMP protocol guarantees the segment will issue only
one join for each group.
The router periodically queries each of its segments, in order to determine if at least one
consumer still subscribes to a given stream. If no responses occur within a given timeout period
(usually about two query intervals) the router will prune the multicast stream from the given
segment.
A more usual method of pruning occurs when consumers wishing to unsubscribe issue an IGMP
“leave group” message. The router will immediately issue a group-specific membership query to
determine whether there are any remaining subscribers of that group on the segment. After the
last consumer of a group has un-subscribed, the router will prune the multicast stream from the
given segment.
8.1.2 Switch IGMP Operation
The IGMP Snooping feature provides a means for switches to snoop (i.e. watch) the operation
of routers, respond with joins/leaves on the behalf of consumer ports and to prune multicast
streams accordingly.
There are two modes of IGMP the switch can be configured to assume, active and passive.
Active Mode
ROS™ IGMP supports “routerless” mode of operation.
When such a switch is used without a multicast router, it is able to function as if it is a
multicast router sending IGMP general queries.