SFP+ networking compatibility

I’m debating which HL-15 motherboard option to go with… the RJ-45 or the SFP+ version.

I have no experience with SFP+, but I’ve been doing some reading but not really getting a 100% confirmation. Eventually I plan to upgrade my homelab LAN to 10G fiber, but currently I’m still on 1G CAT-6 copper. Can some folks weigh in to confirm or contradict my assumptions?

My plan is… I’m assuming that I could go with the HL-15’s SFP+ option and plug in an SFP to RJ-45 transceiver that would connect just fine to my 1G switches and of course I would only get 1G speeds. But that is fine for now.

I’m thinking this is the better upgrade path for the future. I understand that I could get the regular RJ-45 MB now and upgrade to fiber later via a PCIE NIC or alternatively if I go for the SFP+ version I could basically downgrade in the interim by putting a 1G RJ-45 NIC in until I’m ready. These both seem less desirable.

It’s what I’m planning on

I bought 1 of these 1.25G SFP-T, 1000BASE-T Copper SFP, SFP to RJ45 SFP, for Cisco SFP-GE-T, Meraki, Fortinet, Ubiquiti UniFi UF-RJ45-1G, D-Link, Supermicro, Netgear, TP-Link and More Amazon.com

There seems to be some mixed information about this scheme indicating that the chipsets for some SFP+ networking needs to be smart enough to negotiate down, but I can’t figure if that is a limitation just when connecting two fiber devices rather than having the SFP+ port itself recognize that an SFP transceiver has been plugged into it and reacting accordingly.

I have unifi switches, and I’m going to try that rj45 adapter. I’ll let you know once I get it and can test it.

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Here is my 2 cents…

When I purchased my first server ( supermicro brand) that used 2 SFP+ (10Gbe) and 2 RJ-45 (1Gbe) included on the motherboard. I was debating using an SFP connect with RJ-45, but I decided to use SFP+ DAC cables after watching a couple Tom Lawrence videos.

I uses SFP+ DAC cables from FS.com when connecting servers to servers or servers to a router/switch that support SFP+. You do not have to go to FS.com. I started using the SFP+ DAC cables to do a direct connect from 1 server to another server. As I added SFP+ networking cards to the other servers, I started to incorporate a simple mikrotik router CRS305-1G-4S+IN (Amazon.com). This was not a costly solution to implement.

Today, I have added CRS309-1G-8S+in router to create two separate 10Gbe networks. 1 for my lab servers to use and another network to allow my desktop/laptop to you. I have a USB-C network device that uses SFP+.

I hope sharing my experience helps your decision.


it’s on the switch/router side at a hardware level. The internal asics need to be able to negotiate their speed with the partner on the other end. SOME have been able to dupe this, usually pinning it to say 10GbE and then running 2.5GbE through it from the other end, but it’s not ideal for sure.

Ideally, always SFP+ where available. Worst case you have to purchase a 10Gbase-T Transceiver for one side if the other end of the connection is going to terminate in copper. However, that’s not usually the case, as most 10GbE switches have SFP+ cages rather than copper RJ45.

The reasoning behind sticking with a DAC cable (Direct Attach Copper) is that it removes the middleman that is the fiber components. When you send packets over a fiber connection, up until that point it was an “analog” signal, in the sense that it was purely voltage. Now, you need to convert that signal to light, capture it on the other side, and convert it back to electricity. That costs energy and time, which introduces heat and latency.

In the same vein, using a SFP+ 10Gbase-T transceiver means you have to convert that signal to something RJ45 can digest, and that makes LOTS of heat! I’ve been burned a few times by copper transceivers.


Thank you for this response. I forgot to mention how heat is a factor.

Here is a YouTube vlog link from Tom Lawrence as he spoke about his observation with DAC and RJ45. He even talks about latency and power consumption.


WARNING! I have also gotten these same adapters. YES, They work GREAT! Also, from what I have tested with so far, they work with almost anything. BUT, they get HOT! HOT! HOT! I am talking burn your fingers hot if you do not have them in a part of your rack that is going to get active airflow!

I have actually had them stop working from how hot they get and I needed to get these little heat syncs to put on them to help them release heat faster.

I agree with @Glitch3dPenguin. The unit is going to heat up either the motherboard or the SFP+ cage for the router.

There is a lot of heat conducted to change the signal to be transmitted over the Cat6 ethernet cable. I would urge you to use a SFP+ with DAC cables attached. It will save on the wear and tear from all the extra heat generated.


This is what I did to try to help them stay cool and it actually helps a lot. But in most situations you probably won’t have enough space to do this.

This is the 10Gtek 10GBase-T SFP+ to RJ-45 Transceiver with 2 small heat syncs on it to prevent it from overheating in a space that does not get a lot of airflow.

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