@whitequark @unlambda Wow.
Honestly I didn't even know it was possible to get FCSes in Wireshark because NICs normally drop frames with bad FCS so they don't show up in the capture.
Any time I've needed to troubleshoot FCS stuff I've slapped scope probes down on the *MII bus or the SFP diff pair or used my SFP/RJ45 mid-span tap fixtures then decoded in ngscopeclient