To start things off, I’d just finished switching everything over to the new Ryzen 5500–based server I built. Originally this box was using an Intel X540-T2 NIC, which has dual 10Gb RJ45 (10GBase-T) ports. If you’ve ever run 10 gig over twisted pair, you already know those things run hot. Really hot.
I had another NIC kicking around that uses SFP+, and figured if I could find some cheap fiber transceivers it might be a better way to link this machine up to my switch — especially from a heat and power perspective.
I ended up grabbing a pair of Avago-branded SFP+ transceivers and a fiber patch cable for $12.95 shipped on fleabay. Hard to argue with that. They’ve been working great so far, and they run MUCH cooler than the 10GBase-T setup. Like… not even close.
Out of curiosity, I plugged my cheap-o 4x 2.5Gb / 2x 10Gb switch into a Kill-A-Watt to see what was happening. One of the fiber SFP+ modules adds a little over a watt. The 10GBase-T SFP+ module I have? More like 3–4 watts, and that’s just sitting there at idle. Multiply that across ports and uptime and it adds up fast. No wonder 10GBase-T gear runs warm.
Anyway, here’s something neat I didn’t know before today: Linux can tell you all about your installed SFP modules. And not just basic info — actual live diagnostics.
In my case:
sudo ethtool -m enp3s0f0 — See the output of this @ the end of this post.
That command dumps the module’s EEPROM and diagnostic data. You get vendor info, part number, serial number, connector type, supported link modes, wavelength, and cable distance ratings. But you also get live telemetry.
Things like:
Module temperature
Supply voltage
Laser bias current
TX optical power
RX optical power
Alarm and warning thresholds
Which means you can answer questions like:
Is my transceiver overheating?
Is the RX light level getting too low (dirty fiber, bad patch cable, failing optic)?
Is the laser bias current unusually high?
Is anything drifting toward its warning thresholds?
I know very little about fiber compared to twisted pair, but this was pretty eye-opening. From that single command I learned my modules use LC connectors, I’m running 10GBase-SR with an 850 nm wavelength laser, and the link is rated for up to 300 meters on OM3, 80 meters on OM2, and 30 meters on OM1. In other words, short-range multimode optics — not single-mode.
And I can see that mine are currently sitting around 35°C, well under the 80°C warning threshold, with healthy TX/RX power levels and no alarms triggered.
That’s honestly pretty awesome.
I always assumed optics were black boxes. Turns out you can actually get a lot of data from them!
ben@blackbox:~$ sudo ethtool -m enp3s0f0
Identifier : 0x03 (SFP)
Extended identifier : 0x04 (GBIC/SFP defined by 2-wire interface ID)
Connector : 0x07 (LC)
Transceiver type : 10G Ethernet: 10G Base-SR
Encoding : 0x06 (64B/66B)
Rate identifier : 0x00 (unspecified)
Length (SMF) : 0km
Length (OM2) : 80m
Length (OM1) : 30m
Length (Copper or Active cable) : 0m
Length (OM3) : 300m
Laser wavelength : 850nm
Vendor name : EMULEX
Vendor OUI : 00:17:6a
Vendor PN : AFBR-709SMZ-ELX
Vendor rev : G4.1
Option values : 0x00 0x1a
Option : TX_DISABLE implemented
Vendor SN : AD1440302AP
Date code : 141010
Optical diagnostics support : Yes
Laser bias current : 6.486 mA
Laser output power : 0.5858 mW / -2.32 dBm
Receiver signal average optical power : 0.4644 mW / -3.33 dBm
Module temperature : 34.86 degrees C / 94.74 degrees F
Module voltage : 3.3191 V
Alarm/warning flags implemented : Yes
Module temperature high alarm threshold : 85.00 degrees C / 185.00 degrees F
Module temperature high warning threshold : 80.00 degrees C / 176.00 degrees F