"Errors in" when jumbo frames are enabled

[sebeksd]

Hi,
First of all I like to clarify that I'm quite inexperience in networking, I recently bought and configured Celeron J4125 with 4xIntel i225-V (2.5 gig) with OPNsense (v22.1.5).
Of course as always there was ton of problems but I figured out most of it.
There is one thing that is strange and I have no explanation for it, so maybe here on forum someone will be able to help me with it.

Problem:
I have 4x ports, 3 of them are in bridge mode (switch), and last one is separate WAN port.  I have two machines connected to "Switch" ports, 1-server, 2-PC (through another switch or directly, tested few configurations).
Every time I enable Jumbo frames on my PC (4k or 9k) I'm start getting "Errors In" on that port when I run iperf3 between PC and server. Thing is that everything seems to work fine, jumbo frames are working (tested with ping and also iperf3 bitrate increase). Tested with different PC with different cable. Same thing on 1 and 2.5 speed.
Tested witch laptop using USB network card and same thing happens (even after enabling 3k jumbo).
When jumbo is off then I can run iper3 test without single "Error in" (with lower bitrate result).
Disabling offloading on router or on PC did not change anything.

In network capture (done on OPNsense router) I see 2x "Dup ACK" in 10000 packets captured.

[Vesalius]

My unasked-for 2 cents that won't answer your question directly. After much discussion relative to enterprise switches I use, I have come to the conclusion that Jumbo frames are more problem than they are benefit. For many, they only help when you get to networking at the 25g and above level with big blocks of storage data being moved frequently.

Whether it be multicast or IP routing things get messy and nonstandard pretty quickly and implementations from switches to routers to desktops just get random hard-to-pin-down issues. I liken it to using an 18 wheeler with an empty trailer as your daily driver all the time on the off chance you need to haul massive amounts of cargo on the german autobahn.