We have an architecture like this :

2 supervision computers connected to an automaton that is connected to multiple PLC.

When the computer is directly connected with the automaton, there are no issues but when I'm using a cisco C9300 switch, supervision computer don't work anymore with the automaton even if they still can ping it.

Computers and automaton are in the same vlan.

I have noticed that the switch is returning an error : Unknown Protocol Drops

Port used on the switch :

  • GI1/0/2 => PC 1
  • GI1/0/3 => Automaton
  • GI1/0/4 => PC2

What I have tried so far :

  • Everything in trunk mode
  • Everything in access mode
  • Add voice vlan
  • Add Voice vlan dot1P
  • Add lldp transmit / recieve

UAC and firewall are turned off on both computers.

So, when the automaton is using our network via a switch instead of being directly connected to the computer, it doesn't work.

What am I missing ?

UPDATE 15/05/2020 :

So, the automaton technician failed the configuration.

The computer never communicated with the automaton because he used his own computer for the configuration / test ..

Now that he has corrected it, the computer can communicate with the automaton when it's directly connected and it's also working with a switch when computer and automaton are in the same vlan !

Unfortunately, I have to use 2 differents vlan. Automaton has to be in vlan 100 and computer in vlan 101.

When the automaton use the vlan 100 and the computer use the vlan 101, I can ping but the communication between the computer application and the automaton does not work anymore.

I now suspect that the automaton technician is, again, missing something.

Or, I hope not, the profinet protocol can't be routed ?

UPDATE 25/05/2020 :

So. The automaton technician failed again to configure the automaton properly.

Without a switch, it was working, then with a switch, it was working too .. No reason it could not work with a router for me.

The technician fully reloaded the application and it refreshed the new IP (different vlan / gateway).

Now, everything is working fine !

  • 1
    Does the automaton use IP addresses? If not, you probably can't route.
    – Ron Trunk
    May 15, 2020 at 12:56
  • Yes, the automaton use IP addresses, so it's layer 3 but the technician is arguing that the PLC are on layer 2 which it's true but since the automaton has 2 NIC, one for the computer and one for the multiple PLC, vlan shouldn't be a problem. I'm pretty sure he's done something wrong again. I will keep you updated once it's work
    – Meow
    May 15, 2020 at 15:33
  • 1
    Thanks for taking the time to update. Note that you may post it as an answer then accept it (you can "de-accept" the current approved answer).
    – JFL
    May 25, 2020 at 15:16
  • @JFL thanks for the feedback. This is the less I could do for the community. Ron's answer is actually what helped me the most. There were no way that our high end architecture couldn't communicate, so it leaded to the obvious : Automaton Technician did failed his configuration.. twice.
    – Meow
    May 26, 2020 at 8:32

2 Answers 2


Some older industrial systems have their own, non-standard protocols that do not follow Ethernet standards, even though they "look" like Ethernet. That's why you're getting the "unknown protocol" drops. You need a packet capture to determine exactly what is causing the problem, but there's probably not a lot you can do to change this.

  • 3
    One test might be to see if it works across a dumb hub, or perhaps a simple switch.
    – Criggie
    May 14, 2020 at 21:14
  • 1
    Thanks a lot Ron, I was hoping that we were not having this problem. This project is a BIG one and it would have been a real pain to rethink everything because of this. I have updated the post, we now know that the problem was human related which may be still the case for the new error we're having now. Our IT team was thinking like you, including me but we were sure that our high quality / recent material could handle this. @Criggie yes, that's what we did, it showed that the technician missed something because a networking issue was excluded, even more when he failed with no switch at all
    – Meow
    May 15, 2020 at 10:49

If your Network Devices (PLCs & COMPUTERs) are configured with IP Address & Gateways then Follow the Below Steps:

1)Create a vlan in Cisco Switch & activate with "no shutdown"
2)Adding the Default Gateway IP to the VLAN
3)Configure all Ports as ACCESS & Assign the vlan to all ports
4)Remove Spanning Tree , Qos , Port-Security from connected Ports

If Network Devices do not have Default Gateway Configured then Follow the Below Steps:

1)Remove all vlans and keep the default vlan 1 
2)Configure all Ports as ACCESS & Assign the vlan 1 to all ports
3)Remove Spanning Tree , Qos , Port-Security from connected Ports

Hope, this will be helpful !!!!!!!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.