With SFP+ cable choice matters

March 10, 2021   |   by bgarmon

For the last year I’ve been learning through failure about SFP+ 10GB networking. SFP+ requires matching the correct module with the correct wire with the correct settings on the networking device and the correct direction of the cables for the magic of 10GB networking to work.

For my 10GB network I’m running the Ubiquiti Unifi Switch 16 XG . Over the last year I’ve purchased a number of 10GB SFP+ connectors to use with the switch including:

  1. Ubiquiti UDC-3 10G SFP+DAC 3M
  2. Ubiquiti UC-DAC-SFP+
  3. 10Gtek 10GBASE-CU SFP+ Cable .5M (model CAB-10GSFP-P0.5M)
  4. SFP-10G-CU3M 10G SPF+ Passive Copper Cable 3M
  5. Ubiquiti Multi-Mode SFP+ Fiber Module, 10G (model UF-MM-10G) combined with FS.COM’s 50/125 OM3 Multimode Fiber Patch Cable – 2 M (product ID 41729)

I had not noticed any glaring problems with this mix of adapters until I added a vSAN Cluster to the network. I use a 3-node VMware ESX 7.01 vSAN cluster running on the VMware-unsupported Intel NUC computing device platform. Each Intel NUC is using a Thunderbolt 3 10GB SFP+ adapter to connect to the switch (QNAP QNA-T310G1S). While each Intel NUC is hardware identical and software identical, one of the three Intel NUCs network connectivity was configured with Number 4 above and the other 2 were configured with Number 5. This was mainly due to budget constraints where I ran out of inventory of number 5 and had the number 4 in a drawer ready to go. 10GB is 10GB right? Boy was I wrong.

The symptoms I was experiencing involved terrible file transfer speeds and very slow responses from the VM’s running on the cluster along with vMotion events taking a long time. I spent a few weeks troubleshooting the vSAN and ESXi configuration side of this thinking the speeds were likely due to the hard drives or some misconfiguration on my part. When I exhausted that route of troubleshooting I started thinking about hardware differences, the network cable being the only difference. I started simple and ran the included vSAN Host Network Performance Test. It’s the last column in the screenshot that had me concerned:

Anytime the Copper Wire was used vs the Fiber wire, bandwidth tanked

I broke out iPerf and began running a number of other tests and kept coming up with the same problem: something wrong with the connection from AS2, the single device running the Number 4 network configuration. So I broke out the wallet a purchased another set of Number 5, waited a week for the parts to arrive and swapped out the cable. I re-ran the test and my jaw dropped:

all Fiber using Number 4 config.

I re-ran the iPerf tests and was pleased to see all 3 nodes in the vSAN cluster now exhibit the near 10GB speeds I paid for in the first place.

At this point I’ve eliminated all but Number 2 and 5 from the switch and the network has never run faster.

P.S. When replacing the cables, I kept receiving RX errors in the Unifi Network Controller. It turns out I had the fiber cables plugged in the wrong direction. Make sure you have your ends properly matched.


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