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The original was posted on /r/sysadmin by /u/frocsog on 2024-01-22 08:37:19+00:00.
Hello, I'm just a simple sysadmin in a middle school somewhere in an obscure country in Europe. I've been here (and in the trade) since 3 years, and one of the things I've learned pretty quickly is just how unreliable UTP cables are. Our school building has network cables running in the walls and the attic, and the number of times a mysterious network issue resolved with just changing a cable is mad. Now everytime someone calls me, saying "no net", I immediately check the cables. Well, almost everytime.
Lately, the longstanding extracurricular club "Edison club", which helps technically inclined students develop their skills and interests, built a new club house near our school. It has no official ties to our school, but their leader is our IT teacher, and they are somewhat integrated with us. So, they ask if they can have internet from our network. I say yes, because I'm nice, I don't see safety hazards and I know the principal, my boss, is OK with this. It's not a big job anyway, just putting an RJ45 on the end of their cable and firing up a Wi-Fi router. (they did the cabling from our building to theirs, but their cable connects with our in-built network).
Now, the connection is established, but it's unstable. Strange things happen. One time it works, the other time it does not. In the room where their cable is connected, there is an AP which spreads our own network. I discover that either the AP is working, or the club's router. If both connected physically, they do not work. I fiddle with our network settings, putting them in different vlans seems to work. I think I got the issue and I'm walking home happily. Now, after some days, it's not working again. I'm mad at this point, because what started as a 30 minute extra job is now occupying all my problem-solving skills (luckily there's not much work I have to do, one of the things I love this place), and I'm not even payed extra (as this is basically not my work).
So I start to experiment with putting their network connection behind another router, which I know is working, and it turns out it won't work that way either. Then I discover that our cable, with which they connect, is a crossover cable. It shouldn't matter in the age of auto MDI/MDIX, but what do I know. I've seen strange things. I re-make it, and it gets weirder. Judged on the colors, it looks like a straight-through cable, but it measures as a crossover. I look at the cable, I notice there's no "cat 5", or any other sign printed on it. Some 10 meters of cable, running through walls and the attic. I say to the club: we need another cable. I don't have any UTP cable, but they say the will buy and even replace it. They did it, thankfully it wasn't as in-built as I thought, and now it's working fine. Surprise, they discovered a hidden patching somewhere that I didn't know of.
So, check your cables first, folks. After DNS of course.