A Quick Guide to Unicast, Broadcast, and Multicast Transmissions on Ethernet Networks

Hi everyone, Ken Bourke here.

In this post, I'll explain the difference between unicast, broadcast, and multicast data transmissions on EtherNet/IP network and how incorrectly managed multicast transmissions can cause network congestion and performance issues.

Let's get started by talking about unicast data transmissions.

Unicast Transmissions

A unicast transmission is a transmission that is sent from one device to one device.

Unicast transmissions are an efficient way to send data over an Ethernet network and should be used where possible.

Broadcast Transmissions

A broadcast transmission is a transmission that is sent from one device to every device on the network.

These transmissions use a lot of bandwidth but are essential for the operation and maintenance of an Ethernet network. One example of a broadcast transmission on an Ethernet network is an ARP, or Address Resolution Protocol, transmission. ARP transmissions are sent by a device to associate a an IP address with a MAC address.

Some other examples of broadcast transmissions are DHCP and BOOTP transmissions.

Multicast Transmissions

Finally, a multicast transmission is a transmission that is sent from one device to multiple, but not all, devices on a network.

Multicast transmissions are a very efficient way to send the same information to multiple devices simultaneously. They are used extensively in Logix 5000 systems with redundant controllers, safety partners, or I/O modules with multiple listeners.

When dealing with multicast traffic, its very important to know that the default behavior of a Layer 2 switch is to forward all multicast transmissions to every port that belongs to the local network. This means that multicast transmissions behave like broadcast transmissions which can lead to unexpected network congestion.

To deal with this issue, Rockwell Automation recommends using managed switches with IGMP Snooping enabled. IGMP, or Internet Group Management Protocol, Snooping is a constraining mechanism that monitors multicast transmission to develop a table that lists the multicast groups and devices in the groups. This allows the switch to only forward multicast transmissions to devices that need to receive it.

Wrap Up

Now you know the difference between unicast, broadcast, and multicast transmissions on an EtherNet/IP network and what each type of transmission is for.

You also know why Rockwell Automation recommends using managed switches with IGMP snooping enabled for EtherNet/IP networks.