Received Packets With Error

This is driving me nuts.

I have changed a switch to a new HP 1820-24G, which seems quite a nice switch.

But I started seeing rx packet errors in the stats. Now, there is a fibre involved in the uplink on this, and I had moved the endpoint of that fibre to the loft and put an extension fibre patch lead in line, so naturally, seeing rx packet errors, I assume I have screwed up.

I have spent hours on this, new fibre patch leads, cleaning fibre ends, and so on. No joy. Still packet errors. I am coming to the conclusion the switch is lying to me, especially as, finally, it is getting to the point that it shows 1/3 of all Rx packets are in error, and that would be visible in other ways. Pings are clean, and no signs of issues with traffic apart from the reported errors.

But it is not as simple as Rx packet errors.

First off, I did not have a lot on the switch - an "uplink" on port 24, and a "downlink" on port 22 going to the switch in the loft and APs and a load of other stuff in the house. What was especially odd is that the error count for Rx errors on both (port 24 and 22) stayed the same!

So, I moved the uplink from 24 to 23. The count on 23 started going up but the total of 23+24 was the same as the Rx errors on 22!!!

I did the same the other way, moving 22 downlink to 21, but again the total Rx errors from the downlink port was the same as the Rx errors from the uplink.

This really made no sense, and the error rates were low. If I disconnected the downlink I did not see any errors from the uplink. I had an AP and a laptop connected on another port so could confirm all was working. It seemed to matter for the Rx count on the uplink as to whether the downlink was connected.

I spent ages checking and re-crimping cat5 cables, and cleaning the fibres, and changing patch leads and so on - no luck.

Eventually I decided it was clearly the switch being silly, and went on to the other job - reconnecting my neighbour on port 1 on the switch!. This involved a lot of messing about drilling holes and James crawling around in the loft to see where I was poking it through and so on. Eventually I connected port 1.

Now things changed, the uplink Rx errors went through the roof, but the downlink did not - it was still low, and was no longer the same as the uplink. Port 1 showed no Rx errors. But if I disconnect port 1 then the uplink Rx errors go back as before, quite low. If I disconnect port 1 and 22, then the uplink errors stop completely.

I have to say WTF?

Update: If I send packets with no VLAN tag that are 1500 byte payload (so 1518 total) then no errors. If I send packets with a VLAN tag that are 1500 byte payload (so 1522 total) then errors count up. This is even when the switch is set to allow jumbo frames. A clue is that if I set not to jumbo it says the MTU is 1518 on all ports, not 1522. It is clearly a bug in the switch.