User Documentation

User Manual Managed Switches
96
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
Weidmüller switch.
3.8.1 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.
 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, several
industrial automation protocols, such as EtherNet/IP, 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.