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Old 08-15-2011, 04:12 PM   #1
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Default DHCP and Switches

Hello again guys! It's been a long time

I got a new job at a school district and I want to turn a lot of things around. Before I do that, though, I gotta fix a lot of stuff that has been "Broken" since this school has been built three years ago. I've done almost everything in the past few weeks that people haven't done in those three years, but there's another problem I'm coming up against.

In each classroom we have a teacher computer, three student computers, and a printer with ONLY a NIC. The printer is installed locally through the nic, something I've never seen before. Next thing on my project list is a print server

Anyway, back to the problem. With 5 networked devices and 2 drops, we have to have a small 8 port switch in each room (for the time being, I've set up wireless around the campuses). I don't want to go in every room and set up their computers. I let them do it. Some teachers came in last week and did their room (in addition to this week) - and all teachers had to come in this week because today is their first contract day.

99% of them don't know what that gray box (the switch) is. Some of them will plug the switch in to the wall, plug their student computers in to the switch, THEN they will have a left over cable (usually the printer's). They'll take both ends of that cable and plug them both in the switch. The only thing I can figure is it's querying the server nonstop or taking the rest of the DHCP addresses, because whenever someone does this it brings the whole damn network down. I just started two weeks ago.. but I was told this happens every year, and not just at the beginning of the year.

My question is, what exactly could be happening here? And is there a fix without managing each individual switch?

Thanks,

-Q
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Old 08-15-2011, 08:06 PM   #2
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Default Re: DHCP and Switches

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Originally Posted by Captain Pooka View Post
The only thing I can figure is it's querying the server nonstop or taking the rest of the DHCP addresses, because whenever someone does this it brings the whole damn network down. I just started two weeks ago.. but I was told this happens every year, and not just at the beginning of the year.

My question is, what exactly could be happening here? And is there a fix without managing each individual switch?
I think you're dead on with what is happening. I've done this before by mistake which resulted in two ports having constant traffic as each port tried to figure out what was on the other end. It creates a lot of traffic which does eventually bounce around and it took down my own network. If you use a utility like Wireshark and have it running on a machine connected to the faulting switch, and another closer to the router, you can actually watch this happen.
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Old 08-15-2011, 10:11 PM   #3
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Default Re: DHCP and Switches

I saw some enterprising individuals take down a 135 machine network that way. If I remember correctly, they were trying to convince the main office that they needed upgraded equipment.

Maybe time for a training session with all the teachers? If they want their computers to work, then they'll do good to listen to the IT guy.
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Old 08-16-2011, 12:35 PM   #4
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Default Re: DHCP and Switches

Worst problem ever. It creates a network loop. Happens several times a year at my school district too. A cable will fall out or get pulled out and someone will "fix" it by plugging it back in, but it ends up with both ends into one switch. I have 4 buildings all across town so I have to fly around to try to narrow down which building and then which classroom....a real pain in the butt.
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Old 08-16-2011, 01:30 PM   #5
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Default Re: DHCP and Switches

... and the problem is simply solved by moving the routers/switches outside the classroom and having only terminal boxes in the classroom. And only plug in the terminals being used, which helps prevent unauthorised hook-ups as well.

Sure, the teacher has to make a call every time they want to rearrange a classroom, but that shouldn't be happening all that often anyway.
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Old 09-18-2011, 12:14 AM   #6
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Default Re: DHCP and Switches

Well, with the whole budget cut thing in Texas we have many, many teachers moving around every year. The terminal thing would be a good idea... but not until things settle down.

I was just wondering if there's some network management software out there (And I'm going to take a look at Wireshark) that enables you to see when and where this is happening and be able to cut off the rest of the network from the switch that's looping.

I'm kind of new to networking in terms of software to manage it. All of our backbone hardware is different too, we don't have a bunch of Cisco switches... it's about 5 different types throughout one campus. At least if we stuck with one type I could see if that vendor had some soft of manager that managed all switches instead of having to go to each one indivisually. The problem this thread refers to is not with the backbone switch, it's with the small ones in the room - but I would think there is something that the backbone switch can see that would indicate a potential threat to the network and just cut off that port...

Thanks for all your replies!
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Old 09-18-2011, 01:42 AM   #7
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Default Re: DHCP and Switches

I agree with the taking the switches out of the rooms, when you get the go ahead and things calm down. When the switches are moved out have drops installed into the classrooms. Its what we do on our DoD network. Believe it or not our user are probably less intelligent then your school students.

I also agree with trying to get your backbone to one brand. Cisco is pricey, but they have some really good customer support.
Can you just telnet or SSH into these devices? It makes it a heck of a lot easier if you could.

Question: So when someone loops a local switch it brings down the entire network? Backbone and all or just that classroom?
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Old 09-18-2011, 07:38 AM   #8
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Default Re: DHCP and Switches

It'll take down everything on that switch, and sometimes all the way back to the server.
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Old 09-18-2011, 08:37 AM   #9
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Default Re: DHCP and Switches

I've never experienced that, but then again we have all Cisco equipment and their all outfitted with port security and such.
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Old 09-19-2011, 09:48 AM   #10
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Default Re: DHCP and Switches

It brings down everything. We have three hallways, though, and when this happens I unplug a switch in one of the hallways and see if everything comes back up. At least that way I can narrow it down slightly :P
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