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This might not be a real question, my apologies, but I'm pretty sure it will make your doomsday scenario enjoyment work a little.

My company act as a contractor for a larger company. We need to plug various devices on their internal network. They have a (security) limitation that prevents us from plugging a switch or router to any ethernet jack they have available. (why does that even make sense?)

Now since we do need more than one jack, we asked them to lift that restriction for us. They told us that they did. When we arrived on site, we plugged a single switch (very basic stuff) to the jack, and then the whole world came to an end. They experienced major network failure and received logs of "duplicate ips, duplicate macs" and such and the whole plant came to a grinding halt.

Of course we are accused of being responsible for this mess. My coworkers and I totaly fail to see how this could happen. Do you have any idea of why a single switch can crash a whole network?

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up vote 9 down vote accepted

I agree with a few of the answers: It could have been caused by STP. Depending on the STP configuration on your switch (if it was configured for STP) and their switches, it could have caused their switches to see your switch as the STP root bridge or as a designated bridge, disrupting the normal packet flow as implemented and designed by your client. It may very well have also introduced a switch loop in to their network. It also could have disrupted traffic, depending on which version of STP is in use, by causing an STP topology recalculation when you plugged in your switch.

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Sounds like Spanning Tree perhaps - but if you literally just uplinked a switch using a single port, that isn't supposed to happen.

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Depending on the version of STP in use, any link "coming up", whether it be a host or another switch, will cause a STP topology recalculation, which could very well be disruptive to normal traffic flow. – joeqwerty Dec 17 '10 at 18:00
I have seen this with a single port for the reasons stated. The fix was to turn off STP in the switch being added. – dbasnett Dec 17 '10 at 18:12

The only way it could be you or your co workers is:

  • your laptops have static ips
  • your accidently created a switching loop (In which case if its a large network they should have STP setup)
  • one of your machines has a DHCP server installed.

Otherwise I cannot see how you caused an entire network to fail. If they have the security you say they do this wouldnt not have happend by plugging in a switch. I think you can sleep easily.

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I don't get how the switching loop could have happened... But it does seem like it – Eric Dec 17 '10 at 17:30
It depends on how many ports were in your switch, is it possible someone could of plugged both ends of a network cable into the switch? – JamesK Dec 17 '10 at 17:32
It did not happen. Maybe our switch is faulty but i dont think it is the case either – Eric Dec 17 '10 at 17:36
Maybe your switch grabbed the STP root bridge ID, or you propagated STP traffic across VLANs? – Gerald Combs Dec 17 '10 at 18:55

If the switch you plugged into the network was managed, and also happened to have same static IP address assigned to it as a backbone switch. Sounds like could have added problems mentioned already. Can you attach link of switch or model you brought in?

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Switches operate on layer 2, not layer 3. An IP address conflict would affect management of the switch, but nothing else related to traffic flow. – jgoldschrafe Dec 17 '10 at 18:04
Some switches also operate at L3 as they do routing. So if the switch that was plugged in shared the IP address of, say, a switch that functioned as the networks default gateway... A very big "if" admittedly, but possible. – Hutch Dec 17 '10 at 19:14
@Hutch: Good point. I can see a number of scenarios where introducing a foreign switch to an established network might wreak havoc. I hadn't considered a layer 3 switch problem, but as you say, it certainly is possible. – joeqwerty Dec 17 '10 at 20:57
Not direct example, but my network is 192.168.2.xx. Our firewall/gateway/DHCP is on .2.1. Dell's 27xx switches default to if in managed mode. – Chris K Jan 1 '14 at 10:08

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