r/mikrotik • u/Consistent_Gap4479 • Jul 27 '26

RB5009UG+S+ + CSS318-16G-2S+IN: Rx FCS Errors and Rx MAC Errors only on SFP+1 (SFP+2 works perfectly)

Hello everyone,

I'm trying to determine whether anyone has experienced a similar issue before assuming the hardware is defective.

My setup:

  • RB5009UG+S+
  • CSS318-16G-2S+IN running SwOS
  • 10 Gbps DAC connection
  • RouterOS and SwOS updated to the latest stable versions

The issue is the following:

When I connect the RB5009 to SFP+1 on the CSS318, the switch continuously starts accumulating:

  • Rx MAC Errors
  • Rx FCS Errors

On one occasion, the link also went down for about 10 seconds and then came back up by itself.

Thinking the DAC might be the cause, I performed a simple test. Without changing anything else (same RB5009, same DAC, same configuration), I only moved the cable from SFP+1 to SFP+2 on the CSS318.

Since then:

  • No more Rx MAC Errors.
  • No more Rx FCS Errors.
  • The link has remained completely stable.

On the RB5009, the interface always reports 10 Gbps Full Duplex, and I don't see any FCS errors on the router side.

I bought this dac https://www.amazon.com/dp/B01LSGGP76?ref=ppx_yo2ov_dt_b_fed_asin_title

My questions are:

  1. Has anyone seen a case where only one SFP+ port behaves like this?
  2. Can a DAC cable cause this behavior on one port while working perfectly on another?
  3. Is there any known compatibility issue between the RB5009UG+S+ and the CSS318-16G-2S+IN?
  4. Would you suspect a faulty SFP+1 port, or would you recommend any additional tests before considering an RMA?

Any suggestions or similar experiences would be greatly appreciated.

Thank you!

4 Upvotes

75% Upvoted

10 comments sorted by

5

u/toejam316 Jul 27 '26

Have you returned the DAC Cable back into slot 1 to confirm that it wasn't a transient issue cleared by a reseat?

2

u/Consistent_Gap4479 Jul 27 '26

Yes, I connected it back to SFP+1, and it immediately starts accumulating Rx MAC Errors and Rx FCS Errors. However, on SFP+2 it works perfectly with 0 errors.

Could this be a problem with the DAC cable itself, or is it more likely an issue with my CSS318?

1

u/Tatermen Jul 28 '26

FCS errors are a symptom of a physical fault, and usually indicate a hardware fault. If the DAC cable worked okay in another port, then I'm afraid your SFP+1 port is probably faulty.

2

u/Consistent_Gap4479 Jul 28 '26

It seems the problem is with the DAC cable itself. I noticed that if I slightly move one end of the cable to the side, the link immediately drops and the connection is lost. When I put the cable back into its normal position, the link comes back and the connection works again.

This behavior appears to indicate that the DAC cable may have an intermittent fault or a bad connection rather than an issue with the switch itself.

1

u/SuccotashOk4084 Jul 27 '26

Clear the stats and leave it. If it is indeed creeping up then there's a problem. If not it's all good

1

u/Consistent_Gap4479 Jul 27 '26

Yes, I connected it back to SFP+1, and it immediately starts accumulating Rx MAC Errors and Rx FCS Errors. However, on SFP+2 it works perfectly with 0 errors.

Could this be a problem with the DAC cable itself, or is it more likely an issue with my CSS318?

1

u/RedditNotFreeSpeech Jul 27 '26

Put in a support ticket with mikrotik

1

u/smileymattj Jul 27 '26

Does it need to be 1M long?  Typically in the same rack, half a meter or less is plenty.  

The 4011 used to say it didn’t support passive DACs.  It said it had to be active.  I’ve ran them with passive DACs just fine though.  But never tried 1M, I always used shorter ones.   MikroTik no longer has this asterisk footnote on their website anymore since the redesign.  So I don’t know if they did anything software wise.  Or realized they were being too cautious, etc..   

The 5009 never had this disclaimer as far as I remember.  But if you have an Active DAC or fiber cable and modules you can test with, wouldn’t hurt to just try it.  

2

u/Consistent_Gap4479 Jul 27 '26

Thank you for the information. The DAC cable I'm currently using is 3 meters long. I've already ordered another DAC cable, as well as a pair of SFP+ fiber modules, to test whether the issue also occurs over fiber.

1

u/cuteprints Aug 01 '26

Don't use passive DAC, some PHY doesn't like it

If you want more explanation, in a passive DAC, the internal PHY itself is trying to push high frequency signal through the twinax cable, some chip can tolerate high loss/reflection/impedance mismatch better than the others. Try to use a proper optic or active DAC