(A note to mikrotik employees: the cause of this seems to me to not be a configuration issue so I've filed an official support request as SUP-38183, previously SUP-35938. The request has been unanswered for over a week so I am posting here.)

I've got a CRS328-24P-4S+ with 4 suspicious ports. I've updated to 6.48 and "/system check-installation" passes. Nonetheless I see < 1Mbps in iperf3 tests between two systems connected using ports 9, 10, 11, or 12. I have reset the device to stock configuration.

Attached is configuration detail and test summary. The two systems I'm using for the tests are named nt-1gbaset-c and nt-1gbaset-d. I execute the throughput test detailed at the top of the document, then move ports as indicated per scenario. You can see that when the systems are in ports 9,10 and then in 9,18 bandwidth is significantly reduced. Full bandwidth is restored when the test is run after moving to ports 17,18.

Does anyone have insight into the cause of this problem and resolution of it? Thanks.

Have you contacted tech support?

Support has replied and examined a supplied RIF file since I posted this. Here is the response:

> There are FCS errors accounted on both ether10 and ether11 ports, they are usually caused by cable or port damage. To be sure, please, recheck the connection with different cable and after switch reinstallation with Netinstall. If the problem remains, it is likely a physical damage on those specific ports.

So I'm off to find a Windows machine to do that Netinstall.

Try to downgrade to 6.47.8 and i think that your problem will be solved.

This has turned out to be a hardware failure. Mikrotik support determined this when I supplied a RIF file and analysis of it showed "FCS errors" on the ports in question. I've contacted the vendor since warranty exchanges are handled at that level.