I have a CRS317-1G-16S+RM. It's running latest ROS and SwOS. SwOS is my default. Switch works great when running 10g fibre to various Mellanox cards with a few different SFP+ optics.

I'd like to connect a few copper wired clients using 2.5g and 5g links. So I added a pair of fs.com "Cisco SFP-10G-T-X Compatible 10GBASE-T SFP+ Copper RJ-45 30m Transceiver Module" and connected the clients with short cat6 lines for testing.

They negotiate the expected speeds but the performance testing is WAY off in one particular way - uploading from copper clients to the fibre connected server runs at wirespeed (good). Downloading runs about HALF of the wirespeed (bad). 5g, 2.5g and 1g all exhibit the same issue.

10g fibre (I have no 10g copper cards) runs at full speed in both directions (at the same time!) in both file transfers or synthetic IPERF testing.

The FS.com transceivers report themselves to the switch as "850nm multi-mode fiber" with "10G" link speed no matter what the client is connected at. I presume this might be the root cause of the problem - the switch doesn't know the link is much smaller than 10G.

Is the CRS317-1G-16S+RM multi-gig aware/capable? Will switching to Mikrotic S+RJ10 SFP+ units solve my issue?

Hi,

I can confirm that the FS SFP-10G-T modules do not support NBase-T (2.5GBit/s and 5Gbit/s). The Mikrotik S+RJ10 module does support NBase-T but your transfer speed results will only improve after you have enabled Flow Control (for Rx and Tx traffic) on the corresponding SFP+ port in your switch.

I can confirm that the FS SFP-10G-T modules do not support NBase-T (2.5GBit/s and 5Gbit/s).

It's quite weird - they sort of do. There's no problem negotiating a 2.5/5G link with the cards, but what they don't do is inform the CRS317 (or my QNAP QSW-M408S for that matter) that the link isn't anything other than 10G. So I suspect the hardware chip does, but their firmware is lacking.

I've since reproduced the same 100%/50% issue with a 1g connection. Forcing the switch port to 1g FD results in no connection at all.

On my Asus XG-U2008 switch, the 2.5/5G cards will only negotiate a 1G link - there's no NBase-T support at all. On the upside, it does work at expected rates so that's something I guess.

Nevertheless, I've got a S+RJ10 rev2 winging it's way to me now to test with.

Update!
Got my hands on a Mikrotik S+RJ10 module and did some more testing.
As indicated by @lawe, Flow Control is absolutely essential.

1G - problem resolved, wire speed in both directions.
2.5G - problem resolved, wire speed in both directions.
5G - nope. Issue unchanged. Module is reporting 5G speeds in the switch UI, flow control is ON, same pathetic download performance from 10G servers.
Force 2.5G in the NIC, switch reports 2.5G in UI; wirespeed in both directions - almost 100% more download wirespeed than if it autonegs to 5G (2500Mbps instead of 1500-1700).

Firmware bug perhaps?

Nevertheless, I've solved the problem with a QNAP QSW-2104-2S. 10g SFP from the CRS317-1G-16S+RM to it, all the ports easily maxxed at whatever speed they negotiate in either/both directions.