RUGGEDCOM ROS
User Guide
Chapter 5
Setup and Configuration
IGMP 203
Figure 150: Example – IGMP In Operation
1. Producer 2. Membership Queries 3. Membership Reports 4. Host 5. Multicast Router
One producer IP host (P1) is generating two IP multicast streams, M1 and M2. There are four potential
consumers of these streams, C1 through C4. The multicast router discovers which host wishes to subscribe to
which stream by sending general membership queries to each segment.
In this example, the general membership query sent to the C1-C2 segment is answered by a membership report
(or join) indicating the desire to subscribe to stream M2. The router will forward the M2 stream to the C1-C2
segment. In a similar fashion, the router discovers that it must forward stream M1 to segment C3-C4.
A consumer may join any number of multicast groups, issuing a membership report for each group. When a
host issues a membership report, other hosts on the same network segment that also require membership to
the same group suppress their own requests, since they would be redundant. In this way, the IGMP protocol
guarantees the segment will issue only one membership report for each group.
The router periodically queries each of its segments in order to determine whether at least one consumer still
subscribes to a given stream. If it receives no responses within a given time period (usually two query intervals),
the router will prune the multicast stream from the given segment.
A more common 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 unsubscribed,
the router will prune the multicast stream from the given segment.
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 prune multicast streams accordingly. There are
two modes of IGMP the switch can be configured to assume: active and passive.
• Active Mode
IGMP supports a 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.
• Passive Mode
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