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Switches are dropping traffic

Posted on 2008-10-08
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Last Modified: 2012-05-05
I have a set of 3 3com Super Stack switchs model 3870. They are configured as a single stack.   my issue is that they seem to dropping traffic all of a sudden. we have been having an issue with our websites losing connections to the sql server. If a ping anything on our network with the -t option it will be fine for a while and all of a sudden will drop a bunch of packets, and then it is fine for a while.


Does anyone have any ideas on how I could diagnos this problem? I updated the switches to the latest firmware. it hasn't made any difference.

I am wondering if one of my switches might be going bad? if so how do I figure out which one?

thanks

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Question by:mattolan
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by:ctprof
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is this your only network closet that contains 3 network switches?  It sounds like you have your switches stacked on the backplane and then maybe one of them plugged into another one on the front causing a loop.
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evan021702 earned 250 total points
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Yes, I would definitely check for a loop first as well.  Secondly if the switches offer any type of console or web access, check to see if there is one switch that may be receiving a lot of CRC errors.  Typically this means either a device plugged into the switch is bad or the switch or switch port itself is bad.
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by:mattolan
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this is our only wiring closet. 3 stacked switches. nothing has been added or changed in the last few days. and there is nothing plugged directly into the switches that should cause a loop. each port is connected to a single port in different offices around the building.
 the switches do offer web access, but i don't see any kind of obvous logging in them to check for errors.  
if a bad device is plugged in should if affect the whole network or just the port it is plugged into?
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by:Soulja
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How many devices do you have connected to the switches. Do you have vlans set up? Could be too much broadcast traffic.
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by:ctprof
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You should be able to log into those 3coms and show your port status to determine if there are any excessive errors on any of the ports.  It is possible that a bad device could be the culprit.  Also, is your network segregated from the rest of the world? i.e. Do you have any 3rd party companies connecting to your network?   How long ago was the last device added to your network?  
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by:mattolan
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this is our only wiring closet. 3 stacked switches. nothing has been added or changed in the last few days. and there is nothing plugged directly into the switches that should cause a loop. each port is connected to a single port in different offices around the building. the 3 switched are basicly each full (but not all offices currently have computers in them about 100 active IPs at the moment), nothing new has been added lately (1 month). I don't have any vlans set up.
 the switches do offer web access, but i don't see any kind of obvous logging in them to check for errors.  I shows a summary of the ports and they all say 0 errors.
We are segregated from the outside world it is just "us" on these switches
if a bad device is plugged in should if affect the whole network or just the port it is plugged into?
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by:ctprof
ctprof earned 250 total points
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If a bad device is plugged in.. it is possible that it could take down your network.  Here's a couple of instances that I have experienced that resulted in similar problems that you are having with your network.  We have a device that is plugged into two computers with a hub in the middle.  because this hub was in a room instead of a network closet, it got overlooked.  Somehow, we ended up plugging that hub into the switch twice causing a loop and making the network go crazy.   Another instance of off and on network traffic was caused by a network switch that wasn't flushing out it's arp table and somehow applied the same ip address to two different mac addresses.  That one was almost impossible to find.  
You might just try rebooting the switches.. but if you have already done this.. flush the arp tables.
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by:evan021702
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If rebooting did not work you may just want to check to be sure that there no fans going out or the power supply/UPS that the switches are plugged into are not having any issues.  
How often do they go down, every couple of minutes, seconds, etc?
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by:mattolan
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ok. I think the problem was not the switches at all. going on the idea that it may be a bad device I went around the office disconnecting every switch, hub, wireless AP and any other devices I could find that might cause an issue untill the network returned to normal. I then started to plug things back in one at a time untill the issue showed up again.

The issue seems to be caused by our wireless access points. we have 4 of them in the building. it seems that if the middle 2 or turned on so that the 4 overlap coverage areas the network falls apart. if I unplug any 2 so that there is no overlap. things are fine. like they are talking to each other causing a loopback maybe? what I really don't get is that these access points have been on the network in their current locations for about 3 years with no changes and with no apparent issues. I don't understand why all of a sudden they are causing problems?
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by:evan021702
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Are all of them wired back to the switches or do you have one/two of them wired and the others are repeaters?
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by:mattolan
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they are all wired directly back to the switches
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by:evan021702
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I don't quite understand why all of a sudden you would have this issue unless one of them is spoofing the mac address on the other.  What I would do is keep two of them as access points, then make the other two repeaters and uplug them from the switches.  You will still get the same coverage, but eliminate the need to have them plugged in.  
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