Is Ethernet port blinking really useful?

Question

^{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.

`while we don't for other interfaces ` I wish other interfaces had status LEDs. That'd be awesome! — Fake Name, Feb 07 '19 at 20:41
@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 08 '19 at 08:42
They are extremely important, as seen in this (fictional?) story: [Coding machines by Lawrence Kesteloot](https://www.teamten.com/lawrence/writings/coding-machines/) — Mr47, Feb 08 '19 at 10:29
I can ask another question: If the LEDs weren't useful, would manufacturers continue to spend money including them on their products? — Jonathon Reinhart, Feb 08 '19 at 16:37
@forest They used to make [computers that show memory accesses](https://i.stack.imgur.com/mOjgQ.gif) and I remember this past year seeing a kit that has individual LEDs on the DIMMs that show which bank is being accessed - but I can't figure out where I saw it (after 15m of Googling), so the above image will have to do. — Rob, Feb 09 '19 at 15:54
@forest - pinging you since I only get one ping per comment (if I recall correctly). (I may clean-up by removing this comment later.) — TOOGAM, Feb 09 '19 at 19:24
@Mr47 that was one of the more disturbing and awesome things I've read on the internet in a long time. — kingsfoil, Feb 10 '19 at 04:44
Comparing the ThinkPads X200, X220 and X240, one can see that there are fewer indicators on each model. My X220 only has WLAN, BT and HDD LEDs. The X220 also has a blinking WLAN indicator and one for the battery. I presume that most things are visualized in software these days and consumers don't really care about troubleshooting. — Martin Ueding, Feb 10 '19 at 14:07
If you have to ask this question, you've never really worked with networks at all. Those lights are worth their weight in gold. Actually, they don't weight much at all. They're worth a lot more than their weight in gold. — J..., Feb 10 '19 at 20:11
@Mr47 Thank you for the best/worst lunchtime read ever... back to my computer. — Dustin Kreidler, Feb 11 '19 at 20:32
Once I had learned the habits of my network (2 servers, 45 desktops, a dozen non-pc networked devices, 3 fiber lines connecting buildings), I could see at a glance the activity level. It was visually *obvious* if a pc had gotten a virus that was trying to spread, if Pete was downloading p0rn again, or if Mary's network cable was damaged again and causing packet losses. The patterns, tempo and synchronization of the light tell a very verbose tale. I could even see if someone entered the warehouse building, from the IP cameras triggering from movement. — PcMan, Aug 29 '21 at 19:53

Zac67 · Accepted Answer · 2022-02-03T15:02:57.590

61

Yes, blinkenlights are your friends! 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 (sometimes 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 cable end is in the same room - that situation is rather easy to diagnose, even without link and traffic lights. With Ethernet, the far side can be in another room, another building, or even in another city. Twisted-pair cabling is mostly limited to 100 meter, but fiber can cover 100 kilometer or even more. You'll be glad to have a quick, local info light.

Note that the traffic blinking frequency is very often significantly reduced for the human eye - with a potential frame rate of more than 140,000 Hz just for 100M Ethernet, indicating each single frame wouldn't make sense. Well-designed devices attempt to signal various port load states by frequency (like constantly lit for 80-100%, fast blinking for 50-80%, slow for 5-50%, intermittent for 1-5%) but there's no standard sadly.

edited Feb 03 '22 at 15:02

answered Feb 07 '19 at 11:32

Zac67

81,287
3
67
131

I bet they can, but still there are no lights for other data-transferring interfaces like usb, hdmi, thunderbolt or whatever. Why would this be useful only for ethernet? – Stanowczo Feb 07 '19 at 11:35
It'd be useful for other interfaces as well, but due to them being less complex it's not as important. – Zac67 Feb 07 '19 at 12:12
24

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 07 '19 at 16:41
19

@nigel222 Ethernet *twisted pair* cabling can be up to 100 m. Ethernet *fiber* cabling can be *up to 100 km*. – Zac67 Feb 07 '19 at 20:12
Nobody using CB Radio era coax anymore ? – mckenzm Feb 07 '19 at 22:03
2

@mckenzm For Ethernet? I hope not. 10BASE2 sucked big time. – Zac67 Feb 07 '19 at 22:05
14

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 07 '19 at 22:48
5

@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 08 '19 at 13:52
@Zac67: How about MoCA or G.hn then? (Although admittedly they're not really Ethernet, just Ethernet-compatible at L2...) – user1686 Feb 10 '19 at 15:25

score 29 · Answer 2 · answered Feb 07 '19 at 19:21

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.

score 9 · Answer 3 · answered Feb 09 '19 at 04:13

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.

Or there was the time we made a loop and noticed that all the lights were steadily on with no blinking... — Ward - Trying Codidact, Apr 17 '19 at 05:53

score 4 · Answer 4 · 2019-02-07T11:50:30.717

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.

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

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.

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

edited Feb 07 '19 at 11:50

answered Feb 07 '19 at 11:39

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 07 '19 at 12:16
@Stanowczo what do you mean by the ports themselves? – Feb 07 '19 at 12:17
I mean the hole with connectors into which you plug in the cable. – Stanowczo Feb 07 '19 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. – Feb 07 '19 at 12:25
Do you find this useful for consumer products e.g. laptops? This whole question came to my mind because of my work-mate's replicator docking station, which annoyingly keeps blinking me in the face. – Stanowczo Feb 07 '19 at 12:32
@Stanowczo questions about computers or hosts are off topic on Network Engineering. – Feb 07 '19 at 12:34
I think this is a question about computers' network interface, not about computers in general, but well - I might have misunderstood this forum's subject. Thanks for the discussion anyway. – Stanowczo Feb 07 '19 at 12:38
1

@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 09 '19 at 19:21

score 1 · Answer 5 · answered Feb 10 '19 at 12:19

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.

Is Ethernet port blinking really useful?

Is Ethernet port blinking really useful?

5 Answers5