My questions are on the bottom, feel free to skip my debugging story if you're in a hurry.


I started to check a network spanning tree status. All switches are connected on fiber, through SFP ports with transceivers.

By chance, I've stumbled upon a situation that confused me. Let me picture the situation for you guys:

I found that a switch was indicating that one of its SFP ports was down. It should be up. I checked the other link counterpart switch, and the corresponding SFP fiber link was indicating to be online. For the same fiber, one switch on one end was saying the link was up, and the other switch on the other and was saying the same link was down.

That got me confused. I'm talking about 2 switches, one of them is a 3COM 4500 with almost my age, and the second one a HP 1920-24G, both using RSTP. I wondered whether the 3COM disabled its port entirely on a situation regarding spanning tree discarding ports and so on.

I couldn't find any switches that match this situation to compare. So I went on field to check things out.

Indeed, one switch had its link OK, LEDs flashing, browser manager saying link was up, but the other switch on the same fiber was indicating a down link. So I disconnected both ends (they're not too far) of the LC connectors on each switch and used a visual fault locator to check the fiber integrity.

The link that was down showed good visible result on both cords (Rx/Tx), as my equipment could not measure attenuation, I only had my personal binary judgement based on a red laser blinking. The other end, which was up, showed visible results as well, but one of the cords, during the light probe, showed a strange red light from their laser output, as if some of the connectors had some micro dust on its top (showed a round red dot with a "blurish hole" not centered not showing light). I can't resemble which one of them it was by now (Rx/Tx). After rubbing the conector on my shirt, I tested again and got apparently a good visual response.

After that, I reconnected everything back, and the link status that was down went online back again. I checked another switch, a third one, far away, that I have not even mentioned here, and one of its SFP ports was disabled on the RSTP, as I was expecting, according to the local network topology, that link was the backup one, and it became the backup as soon as I reestablished the offline oficial link.

Unfortunately, about half an hour later, when I was telling all that to a maintenance supervisor, to keep him informed, I was going to show the switch with all the SFP ports OK, but since Karma happens, the link was down again.

We are going to replace the SC/LC patch cord connectors on both ends and see if the problem ends.

So my final questions are:

1) Does a SFP port LED indicator goes on if the transceiver receives packets? Or when the transceiver receives AND transmits packets?

I ask that because since we use two channels, one to transmit and other to receive, maybe one switch had a transmit link OK (receiving link OK on the other end) and the other receiving link was bad (transmitting link on the other end bad). If a SFP link, for a switch, is online when its port is receiving, regardless of transmission (Rx working, Tx don't know), my assumption is correct based on what I saw today.

2) What is probably causing the link to be faulty sometimes?

That link was good for almost 2 years. Maybe dust? Maybe faulty transceiver? Maybe bad optic patch cord? Maybe the SC pigtail fusion? Maybe environment (hot climate, low concentration acid fumes) ? I can guarantee no one has ever touched that connection since the startup of the link.

2 Answers 2


For the same fiber, one switch on one end was saying the link was up, and the other switch on the other and was saying the same link was down.

That happens when Switch A (Rx) receives a signal from Switch B (Tx), but Switch B (Rx), for whatever reason (bad fiber, bad SFP, dirt/dust, etc.) does not receive a good signal from Switch A (Tx). Switch A would show the link up, but Switch B would show the link down.

Does a SFP port LED indicator goes on if the transceiver receives packets? Or when the transceiver receives AND transmits packets?

That depends on the switch. Some switches have a solid link light, while others will have both transmit and receive lights that flash as traffic is sent and received, and some switches may have other ways, such as a single light that flashes for traffic in any direction.

What is probably causing the link to be faulty sometimes?

Without more information, that is really unanswerable. For example, fibers develop micro-cracks in the glass fiber, and they can get bad enough to cause problems. There are specifications for maximum pulling tension and minimum bend radius that, when exceeded, can ruin a fiber. That can vary by fiber manufacturer, but absent any manufacturer information, the unloaded minimum bend radius is 10 times the outer diameter of the cable. It could be more severe than that, and adding any load would require the minimum bend radius to be expanded (a lot).

  • Ron, indeed that was the case. Testing with a brand new fiber patch cord, I was plugging only one SC connector at a time, and verify that the only SFP port LED indicator goes on if the switch receives signal. By alternating the SC connectors plugged in, both switches alternated status indicator. Now I know they behave this way. I'll be monitoring the link closely now. Thanks for the info.
    – MZanetti
    Nov 5, 2019 at 12:17

There's no general rule for that. Different vendors do things differently, even with different device series.

Most often, there are two indicators per port: one indicating link status (also lighting up briefly when an inserted module is activated) and the other indicating activity - either direction.

Note that both link status and activity are from the port's perspective: the link status is lit even when spanning tree blocks the port. Activity may just indicate BPDUs and LLDP/CDP frames on an STP blocked port.

To really find out what's going on you'll need to log on to the switch's console and inspect port status there and look at the logs. With optical links, it is possible to have a link working in one direction only, so you'll need to look at each sides.

Most 850 nm SFPs (10BASE-FL, 1000BASE-SX, ...) can be inspected visually by glancing into the transmitter size - part of the transmitted light leaks into the visible spectrum. Using an electronic camera, you can see a lot more light. 1300 nm light is generally invisible and requires special equipment for detection.

Links can be degraded by dirt, damage to the endfaces of the patch cables or of the panel jacks, damaged, bent or stretched cables, ...

  • Those switches only got one indicator. And for sure, now, they state to be on and blink sometimes even on a STP blocking state, as you have said. Thanks!
    – MZanetti
    Nov 5, 2019 at 12:19

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