43

This question may not look as an important one and in fact it is just out of curiosity.

But to the point:

Is Ethernet port blinking really useful?

Ethernet ports usually have two light indicators of activity (blinking leds). Yes, I know that they indeed indicate activity, but why do we need that for Ethernet ports while we don't for other interfaces? Is there any official explanation for that or is it just a matter of tradition nobody cares about? Or else?

Looking forward to see your knowledge or ideas on the topic.

  • 29
    Always start troubleshooting at layer-1. – Ron Maupin Feb 7 at 15:11
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    while we don't for other interfaces I wish other interfaces had status LEDs. That'd be awesome! – Fake Name Feb 7 at 20:41
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    @FakeName Why stop there? Let's add blinkenlights for interrupts and for memory accesses. I'd love to see each CPU cache line have its own LED! – forest Feb 8 at 8:42
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    They are extremely important, as seen in this (fictional?) story: Coding machines by Lawrence Kesteloot – Mr47 Feb 8 at 10:29
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    @Mr47 That was... unsettling. – You Feb 8 at 15:48
59

Yes, the lights can be very helpful when diagnosing problems - especially when dealing with non-managed switches or remote diagnosis with unexperienced users.

Rule of thumb:

  • no link light = layer 1 (cable/port) problem
  • link light but no traffic = layer 2 problem (or higher) - VLANs, STP, port security, IP subnet mismatch, ...
  • traffic light constantly on but little useful traffic = bridge loop (or constant collisions with a repeater hub), possible duplex mismatch

Many switches can signal multiple infos (possibly using a toggle button): link speed (none/10M/100M/1G/10G), half/full duplex mode, slow/fast traffic, PoE available/applied, high error rate, inserted transceiver accepted/rejected and more. Most of these are very helpful when something doesn't work.

With other (shorter-range) interfaces, most often the other side is in the same room - that way, it's much easier to get by, even without link and traffic lights. With Ethernet, the far side can be in another room, another building or even in another city. You'll be glad to have a quick, local info light.

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    Ethernet wiring can be up to 100m, buried in the structure of a building, gnawed by rats, dribbled with sewage leaks, melted by plumbers, etc. Unreliability goes with the territory! At my workplace many of the computers are in a clean area. It takes about 5 minutes struggling into a clean-room suit before you are allowed in there, another 5 minutes disrobing to come out. If you form the opinion that the network might be faulty, it's incredibly useful to be able to check if there's a link and if there's any data passing on it, without leaving that area! – nigel222 Feb 7 at 16:41
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    @nigel222 Ethernet twisted pair cabling can be up to 100 m. Ethernet fiber cabling can be up to 100 km. – Zac67 Feb 7 at 20:12
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    @mckenzm For Ethernet? I hope not. 10BASE2 sucked big time. – Zac67 Feb 7 at 22:05
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    As an example, I followed the blinking lights to trace a denial-of-service attack through an office, and found a computer with an unsecured NTP server. – Mark Feb 7 at 22:48
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    @nigel222 from my industry I'd like to add, Ethernet can be stuck inside of moving machinery without the proper high flex cabling, soaked with hazardous chemicals without the proper jacket, have its jacket rubbed off by improperly designed machinery, be partially sheared off when it was rammed into something accidentally, have its X-Code sockets be cut open by electricians forcibly pushing connectors together, flaky cards can be toasted by PoE voltage being shorted to communication pins, networks can be accidentally limited to the wrong speed by using a 4 pin cable in the middle, etc... – jrh Feb 8 at 13:52
27

My very first step in troubleshooting network issues is this:

"Are there winky-blinky lights?"

It's the network port equivalent of asking someone if their computer and/or monitor is actually turned on (and/or plugged in) when they complain about the screen being dark. A dumb question... but its amazing how many times they come back with a dumb answer. Still, those are the best, most easily solved issues.

8

The patterns of blinking lights on a switch can give you a quick overview of network health.

I used to work at a medium-sized company of about 100 staff, 250 PCs, a dozen servers, and the main server room was across the hall from IT's offices. The server room had windows so we could see the racks, consoles, etc., and we could see the status LEDs of the core switch that all the servers and remote switches connected to.

Since we'd glance at the switch and all the other blinky lights many times a day, we'd recognize normal and non-normal patterns. We knew which ports had the busiest and least busy servers, so if we didn't see the right amount of activity, we'd know we should take a closer look.

  • 1
    Or there was the time we made a loop and noticed that all the lights were steadily on with no blinking... – Ward - Reinstate Monica Apr 17 at 5:53
4

The LEDs on Cisco equipment has a function. For instance, below is an example from the Cisco 2960-X Series access layer 2 switch. The LED indications changes from model to model. This is just one example.

You can use the switch LEDs to monitor switch activity and its performance.

Switch LEDs and Mode Button for the Catalyst 2960-X Switch. This figure shows the switch LEDs and the Mode button that you use to select a port mode.

1 RPS LED
2 SPEED LED
3 STAT LED
4 SYS LED
5 Mode button
6 Master LED
7 STACK LED
8 PoE LED
9 USB mini-Type B console port LED
10 USB Type A port
11 MGMT LED
12 CONSOLE LED
13 USB Type A port
14 Port LEDs

RPS = redundant power system—only on switch models that support RPS.

Only on switch models that support PoE.

Only on switch models that support stacking.

enter image description here

The RPS LED is only available on switch models that have an RPS port.

enter image description here

enter image description here

The port and module slots each has a port LED. As a group or individually, the LEDs show information about the switch and about the ports.

To select or change a mode, press the Mode button until the desired mode is highlighted. When you change port modes, the meanings of the port LED colors also change.

enter image description here

enter image description here enter image description here

If your switches are stacked and you press the Mode button on any switch, all the switches display the same selected mode. For example, if you press the Mode button on the stack master to display SPEED, all the other stack members display SPEED.

Even if PoE mode is not selected, this LED still shows PoE problems if they are detected.

Source: https://www.cisco.com/en/US/docs/switches/lan/catalyst2960x/hardware/installation/guide/b_c2960x_hig_chapter_01.html#ID277

  • 1
    Thank you for this answer, but what I' about are ethernet ports themselves, not the devices that use them - here I have no doubts. But ports have led indicators also in many laptops, docking stations, replicators - generally client devices. – Stanowczo Feb 7 at 12:16
  • @Stanowczo what do you mean by the ports themselves? – user36472 Feb 7 at 12:17
  • I mean the hole with connectors into which you plug in the cable. – Stanowczo Feb 7 at 12:20
  • @Stanowczo those are explained in the last two pictures of my post, what the LEDs indicate for actual Ethernet ports. But as i wrote it can vary from model to model. – user36472 Feb 7 at 12:25
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    @Stanowczo : I think this particular site is more about commerical-grade solutions (as opposed to, say, Superuser.com). If your issue is an annoying blinking light, then a bit of electrical tape can often resolve that. – TOOGAM Feb 9 at 19:21
1

Yes, it is useful. You can see interface activity and sometimes based on the blink rate how much traffic is going via the interface, although some systems artificially reduce the blink rate.

Although other ports often don't have a blinking status LED, such a LED could be useful for other ports as well. For example, my Canon EOS 2000D DSLR camera has a status LED that indicates not only Wi-Fi activity but also USB activity. So, I can very easily see when it's safe to turn power off and disconnect the camera from the USB port: if the LED is blinking, files are still being transferred to the computer.

Some USB mass storage devices also have a blinking LED. For example, my USB hard drive has a blinking LED, and even some low-cost USB pen drives happen to have a blinking LED!

Not only that, but you often see the link speed on Ethernet switches from the LED color and which LEDs are on and which are off. I can far more quickly diagnose link speed issues just by looking at the LEDs than logging in the web management console with a password. Also, some cheap switches lack a management interface. If you have a non-smart switch, the only way to verify negotiated link speed in switch-switch interconnections is via the status LEDs.

I wish more devices had a blinking LED. That'd be great e.g. for USB: to quickly verify if it's using USB1, USB2 or USB3 and see data transfer from blink rate.

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