Micrologix Gratuitous ARP

Hello,

I've recently installed wireshark on my DA server (Wonderware). I have a network will alot of micrologix PLC's. Since installing wireshark I can see that the micrologix are constantly sending out gratuitous ARPs. I did a little research on this and it sounds like gratuitous ARP's should only happen on startup of the PLC. But it looks like the micrologix is routinely sending them out. Does anyone know anything about this?

Micro 1100 or 1400, and what firmware?

I think this may be the duplicate IP check. From memory the early firmwares did this every 2 minutes whilst running. Newer firmware has an option to turn this off under the ethernet channel options in RSLogix500

This is a bit of a guess but I think that’s how it maintains its connections.

Hello Saffa, I checked the firmware and it was version 9. So I upgraded it to 15 but I don't see anything related to duplicate ID check options in the Channel 1(ethernet) tab. Also, I'm using Micrologix 1100's.

Firejo. I don't see why a constant gratuituous ARP is required for maintaining connection. ARP in networking is used to update the ARP tables of the switches, OS and is usually only broadcasted once if the device exists. In this case a gratuitous ARP is almost like reverse (but not like a reverse-ARP). The end device sends an ARP so all the switches, OS's etc. can update the their ARP cache. Logically this should only happen one time (on bootup). Since a MAC address normally wouldn't change on a PLC (unless you change the network card or PLC if network is integrated) I don't see why ARP-ing multiple times would be required to maintain connection. I can see it being used if it was checking for duplicate IP address like Saffa had mentioned.

ARP packages are very small compared to what the actual data traffic is but I'd like to try to maintain a clean network as well.

Well it was a guess. I now have it on good authority that the repeated gratuitous ARP’s are for duplicate IP address detection.

I've just had a look in RSlogix and the "turn off duplicate IP address detection" is only available in series B 1400 controllers. I think this was added to make it more friendly for low bandwidth telemetry applications.

So not much you can do for your 1100, but i wouldn't be too concerned really.