User`s manual

VPort 251 User’s Manual System Configuration
Multicast
The VPort 251 supports the advanced Multicast network protocol IGMP, which can greatly
improve the efficiency of network traffic. In this section, we explain multicasts, multicast filtering,
and how multicast can be implemented on your VPort.
What is Multicast?
A multicast is a packet that is intended for “one-to-many” and “many-to-many” communication.
Users explicitly request to participate in the communication by joining an end-station to a specific
multicast group. 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 relevant multicast group. Multicast
group members can be distributed across multiple subnetworks. Therefore, 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. The packets are only replicated
if they reach a network node that links to two or more members of the multicast network.
Transmitting packets in this way makes more efficient use of network bandwidth. A multicast
packet is identified by the presence of 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 Enables the simultaneous delivery of information to many receivers in the most efficient,
logical way.
y Reduces the load on the source (for example, a server) because it does not need to produce
multiple copies of the same data.
y Makes efficient use of network bandwidth and scales well as the number of participants or
collaborators expands.
y Works with other IP protocols and services, such as Quality of Service (QoS).
y There are situations where a multicast approach is more logical and efficient than a unicast
approach. A typical use of multicasts is in video-conferencing, in which high volumes of traffic
need to be sent to several end-stations simultaneously, but for which broadcasting that traffic to
all end-stations would seriously reduce network performance. Besides, several industrial
automation protocols, such as Allen-Bradley, EtherNet/IP, Siemens Profibus, and Foundation
Fieldbus HSE (High Speed Ethernet), use the multicast approach. These industrial Ethernet
protocols use publisher/subscriber communications models by multicasting packets that could
flood a network with heavy traffic. IGMP provides the ability to prune multicast traffic so that it
travels only to those end destinations that require the traffic, thus reducing the amount of traffic
on the Ethernet LAN.
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