User Manual Managed Switches
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reaches group members diverges. To make more efficient use of network bandwidth, it is only at
these points that multicast packets are duplicated and forwarded. A multicast packet has a multicast
group address in the destination address field of the packet's IP header.
Benefits of Multicast
The benefits of using IP multicast are:
•
It uses the most efficient, sensible method to deliver the same information to many receivers with
only one transmission.
•
It reduces the load on the source (for example, a server) since it will not need to produce several
copies of the same data.
•
It makes efficient use of network bandwidth and scales well as the number of multicast group
members increases.
•
It works with other IP protocols and services, such as Quality of Service (QoS).
Multicast transmission makes more sense and is more efficient than unicast transmission for some
applications. For example, multicasts are often used for video-conferencing, since high volumes of
traffic must be sent to several end-stations at the same time, but where broadcasting the traffic to all
end-stations would cause a substantial reduction in network performance. Furthermore, the GOOSE
messages and SAMPLED VALUES defined in the IEC 61850 standard are multicast and use
publisher/subscriber communications models by multicasting packets that could flood a network with
heavy traffic.
IGMP Snooping is used to prune multicast traffic so that it travels only to those end destinations that
require the traffic, reducing the amount of traffic on the Ethernet LAN.
Multicast Filtering
Multicast filtering ensures that only end-stations that have joined certain groups receive multicast
traffic. With multicast filtering, network devices only forward multicast traffic to the ports that are
connected to registered end-stations. The following two figures illustrate how a network behaves
without multicast filtering, and with multicast filtering.
Network without multicast filtering
All ho
sts receive the multicast traffic, even if they don’t need it.
