The need for Managed Switches

In the last year or so I have run into a lot of Braas customers who are Machine Builders or OEMs using Rockwell PLCs, drives, HMI’s, etc.  More often than not these are connected with Unmanaged network switches.  Is this a good thing or a bad thing?  The technology behind that decision is a bit foggy so I want to break it down for laymen.  At the end of this post is a download link with complete details on Ethernet/IP for Rockwell networks if you want more details.

The Text Message Analogy

Let’s compare Ethernet traffic to something we do every day, text message.

  • Most of our text messages are one-to-one SMS messages between two parties.  In the world of Ethernet networking these equate to a Unicast message.  Think about it- Uni “one” -cast.  This is a simple and efficient one-to-one communication process.
  • Now the other end of the spectrum.  Have you ever gotten an Amber Alert?  You’re sitting in a room and everyone’s phone blasts out the alert at one time.  It’s kind of freaky.  In the Ethernet networking world this is a Broadcast message.  Think of it as one to everyone.  Imagine what it would be like if all of your text messages went out to everyone on your contact list every time you sent a message.  The resulting traffic you get from angry friends would probably result in you never texting again.
  • This brings us to the painful realm of group text messages.  I hate those group messages where everyone banters on for hours (do you people have a life!?)- but they do have a purpose.  If you need to get a message out to several people at the same time you can send one group message or you can send out several individual messages.  The group message is much easier than poking out the same message a dozen times.  In the Ethernet networking world this is much like a Multicast message.  Multi “many”- cast.  So as long as everyone in the group behaves the communications is efficient.

How Managed Switches Help

Rockwell PLCs and  networked devices communicate on Ethernet networks via Ethernet/IP protocol.  This is an ODVA standard and open protocol.   The protocol takes advantage of the time saving use of Multicast messaging.  It is very efficient for a PLC to send out one Multicast message asking for IO updates, etc. instead of several Unicast messages.  A managed switch quickly learns how how to manage Multicast data by only sending the traffic through the appropriate network port to get the message to the intended target device.  An unmanaged switch treats Multicast messages as broadcast messages and sends the data everywhere.   In even a medium sized system this floods the network and can cause jittery communications or even stoppages.  Signs of the problem include inconsistent updates of HMI variables, slow response to operator input, jittery motion control or poor vision systems performance.

The mechanism for filtering Multicast traffic is IGMP snooping found only in managed network switches.  IGMP stands for Internet Group Management Protocol and is an integral part of Multicast messaging.  Without IGMP snooping the Multicast traffic behaves like that Amber Alert and it broadcasts each message to every device on the network.  So how frequently is your PLC polling your IO, several times a second?  Every time it polls it floods every port the network with traffic.

A Single Multicast Message with Unmanaged Switches
A Single Multicast Message Filtered with Managed Switches












Its wrong to think that the problem is just a traffic jam in the network switches, it’s worse.  Every time one of your networked device sees one of these packets it has to stop what it is doing and read the packet to determine if it needs to respond or not.  So motion controllers, vision systems, HMI’s, etc. all have to stop what they are doing and inspect many, many messages that are not intended for them.  It’s no wonder they get aggravated like I do with a group text gone mad.

Managed switches to solve this problem have become very affordable.  Moxa offers the SDS-3008 lightly manged switch with IGMP snooping and many other valuable features for under $400.  You can learn more about this switch on Moxa’s web site.

You can also download a brochure here. 


Contact us for volume pricing.


Download ODVA Guide here.  Skip to Appendix A for the recommendations for switches.


Download our own technical guide on IGMP snooping here. 


Please feel free to contact me, Reid Garst (, or Jack Baldwin ( for more information.