ToughNet NAT Router User's Manual
TN-5916 Series Features and Functions
3-18
Multicast
Multicast filtering improves the performance of networks that carry multicast traffic. This section explains
multicasts, multicast filtering, and how multicast filtering can be implemented on your Moxa ToughNet NAT
Router.
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 subnets, 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:
•
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.
•
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.
