Siemens RX1500 User Manual page 596

Chapter 5
Setup and Configuration
The IGMP protocol operates between multicast routers and IP hosts. When an unmanaged switch is placed
between multicast routers and their hosts, the multicast streams will be distributed to all ports.This may introduce
significant traffic onto ports that do not require it and receive no benefit from it.
IGMP Snooping, when enabled, will act on IGMP messages sent from the router and the host, restricting traffic
streams to the appropriate LAN segments.
Example: IGMP In Operation
The following network diagram provides a simple example of the use of IGMP.
Figure 658: Example – IGMP In Operation
1. Producer 2. Membership Queries
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.
564
3
C1 C2
4
3. Membership Reports
P1
M1 M2
2
5
4. Host 5. Mulicast Router
1
2
3
C3 C4
RUGGEDCOM ROX II
User Guide
IGMP