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Lomsor
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Is "forwarding-override" always Hardware Offloaded?

Fri Jul 15, 2022 3:46 pm

Hello I am looking into using Mikrotik Switches for commercial industrial applications.

We haven't done any prototyping yet but are looking into theoretical feasibility.
One of the things we would like to do is forward EtherCAT Datagrams (Packets) trough Microtik CRS with no processing at all and minimal latency and jitter.
For this we have eyed the CRS326-24G-2S+RM and CRS328-24P-4S+RM.

EtherCAT uses Ethernet but is quite different from other packets. It does not specify any target address at all but "broadcasts" datagrams cyclically along a duplex ethernet path that is expected to be a logical loop (no branches, copies or "replies", the datagram should eventually travel back to the master).
It can have an optional VLAN tag though that we would like to use too but that's off topic for now.

Gladly for us I stumbled upon some research that has been done on the matter in the past. This cool (but a little limited in testing) paper even uses Mikrotik devices, but it is using other firmware, not RouterOS: https://www.mdpi.com/1424-8220/22/5/1944

We on the other hand would like to use RouterOS as it offers features that we could use for other things we would like to do with the CRS.

We want to send datagrams ingressing into the switch out a specific other ethernet interface port and do that multiple times for different ports, eventually sending datagrams ingressing into the last ethernet interface port out the first port behind which the EtherCAT master resides.
It seems like the "forwarding-override" mentioned here in the switch chip feature documentation is the most appropriate way to do it.

So here are my questions:
Should this be theoretically possible on Mikrotik CRS Switches/Routers?
and: Is the "forwarding-override" functionality always hardware offloaded to the switch chip or is there a limitation like is seen with the limit of one "bridge interfaces"?
also: How does it work? Does it override other forwarding or mirroring options and if, which have priority? What else is to consider?

Given the limited documentation it sounds like the "forwarding-override" functionality is switch chip exclusive and can't even run on the CPU but that's why I am asking, it's not made clear at all.

I assume other features of that ethernet interface port have to be disabled in order to further optimize for this task, some input on that front would be appreciated too.

As we would like to use this feature to create a "Software Defined Network" as is described in the paper we will also probably use the "/interface ethernet set XX disabled:yes/no" command in order to virtually disconnect EtherCAT Devices if needed on the fly ... this command I have seen being used but it is not documented at all. I find the lack of documentation for some features a bit concerning as it creates a situation where I am unsure if a Mikrotik device has the capability to fulfill a certain role or not.

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