EDS-728 Series User’s Manual
Featured Functions
3-44
The Concept of Multicast Filtering
What is an IP Multicast?
A
multicast
is a packet sent by one host to multiple hosts. Only those hosts that belong to a
specific multicast group will receive the multicast. If the network is set up correctly, a multicast
can only be sent to an end-station or a subset of end-stations on a LAN or VLAN that belong to
the multicast group. Multicast group members can be distributed across multiple subnetworks, so
that multicast transmissions can occur within a campus LAN or over a WAN. In addition,
networks that support IP multicast send only
one
copy of the desired information across the
network until the delivery path that 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 that it:
y
Uses the most efficient, sensible method to deliver the same information to many receivers
with only one transmission.
y
Reduces the load on the source (for example, a server) since it will not need to produce
several copies of the same data.
y
Makes efficient use of network bandwidth and scales well as the number of multicast group
members increases.
y
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, several
industrial automation protocols, such as Allen-Bradley, EtherNet/IP, Siemens Profibus, and
Foundation Fieldbus HSE (High Speed Ethernet), use multicast. These industrial Ethernet
protocols 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.
