Network Dies When One Room Connected

Hi folks! I've got a bit of a puzzler...

On the 3rd floor of our building, we have three computer labs. Each has it's own switch which connects all of the PC's, printers, etc. Each of those three switches then has a CAT6 cable which connects them back to a backbone switch in our main server room on the 1st floor. That switch is then connected to our router/firewall, a SonicWALL TZ-210.

There is one switch/room on that floor which is essentially killing the whole network if it is connected. If I pull power to that switch, everything in the other two labs functions fine. If I connect it, then computers will successfully find the DHCP server and get an IP address, but that's it. They won't be able to log in via Active Directory, they won't be able to ping the router, they will only very intermittently be able to ping each other. Disconnect the room in question and instantly everything else works again.

Clearly, either the switch is malfunctioning/misconfigured or there is a device connected in that room that is doing something it shouldn't. I'm trying to narrow it down, though, by determining what exactly could cause these symptoms.

Any suggestions?

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Slow but sure: First determine if your switch is the problem. Disconnect all your cables from the switch except for the one leading to your network. See if the issue is still present. If not then one by one reconnect the cables and start up your connections on the end of each cable, check your network, before attempting to connect the next, until you find the culprit.

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Fred MarshallPrincipalCommented:
Are these managed switches?  Do you have control over the ports up/down?

If so, I might selectively "down" each port to see what the result might be.  You might start with the link to the main switch.  (If you aren't AT the switch then don't cut yourself off!!)  Then go through all the active client ports until/if you can find one that makes this happen.  Otherwise it's physical plug/unplug as suggested above.

Do the clients on this switch work when it's connected?
Mohammed KhawajaManager - Infrastructure:  Information TechnologyCommented:
Try swapping the defective switch with a working switch to ensure it is not bad wiring causing the issue.
Fred MarshallPrincipalCommented:
If you've not swapped a switch before and don't want to cause disruption, here is a "trick" that I use in that situation:

Neglecting any specially defined ports (!!):

If possible, stack the existing and new switch one on top of the other.
Connect an Ethernet jumper cable between spare ports on each so the switches are connected together.
Move cables one-by-one from the existing switch to the new switch.
If there is a backbone connection, move it first.
(The only interruption should be the time it takes to move the one cable plus a tiny bit of time for ARP table adjustment that will happen).
When all of the cables are moved, remove the jumper cable and the "old" switch will be completely disconnected.

In your case, it would be useful to have a monitor that tells you what the network at the other locations is doing as you move individual cables.
In doing this, it may be useful to do the backbone cable LAST.
Then you might see that one connection being removed makes the difference.
I'm sure there are lots of strategies for this.

Another strategy:
Plug in a 5-port switch to the backbone cable.
Run a cable from that switch to both the existing switch and the new switch.
If this helps then there is something likely wrong with the existing switch.
Move cables from one switch to the other as above.
If disconnecting one cable fixes the problem:
  If connecting that cable to the new switch restores the problem
     the problem appears to be associated with that one cable/host combination.
  OTHERWISE connecting the cable to the new switch does not restore the problem.
     this suggests something wrong with the existing switch port it was connected to.

and so forth .....

Then, if you decide to keep the new switch, it's just a matter of moving the backbone cable directly to it and removing the extra switch added for testing.
IthizarAuthor Commented:
After much testing and tracing, it turns out that there were two runs from the main switch on the 1st floor which were connected to the problem room's switch. This appeared to create a loop or broadcast storm of some sort that was killing network traffic. I am surprised, because I was under the impression that all modern switches were designed to prevent that sort of thing. But as soon as I remedied that, all is well.

Thanks everyone!
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