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How to find a port which is causing network looping on the workstation side Cisco 3560G

Posted on 2013-06-03
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Last Modified: 2013-06-19
Hi

We have Cisco 3560G core switch on our network and we have a mixed flavors of Dlink and HP as slave switches. There are 4 Fiber uplinks ports connected to this switch.
Recently one of the uplink was shut down on the core switch because of the looping on the workstation side.

I have to completely disconnect all the slave switches which were uplinked through the fiber backbone and slowly connected one by one switch to find which port is causing this network looping. Finally I found the port on the switch side.

We have around 1000 network ports and my predecessors didn’t document the cabling numbering properly, and I am not able to find the actual port on the work station side.
Is there a way to find this?

Thanks
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Question by:lianne143
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by:unfragmented
unfragmented earned 250 total points
ID: 39217989
With managed layer 2 switches there are normally a number of STP options.
Enabling STP should protect the network from layer 2 loops.
Other enhancements such as BPDUGuard and UDLD would further protect and alert you to potential layer 2 loops.

Unfortunately without managed switches, these features are unavailable, and your network is at the mercy of the physical layer.  As you have discovered, the only way to troubleshoot a looping network without managed switches is by turning off or disconnecting things until you find the offending device/link.
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by:agonza07
agonza07 earned 125 total points
ID: 39218000
If you have a separate management network for you 3560 and you can still get into it when a storm is happening, you may be able to do a show interface and perhaps see which interface has a high number of broadcasts. May or may not help, but worth a shot.
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by:lianne143
ID: 39218325
My predecessors never left the password for the Cisco 3560G core switch , when they left.
I remember them saying that they have configured STP on this switch.
Is it possible to reset the password on this swith and will i loose any configuration?

Thanks
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by:unfragmented
unfragmented earned 250 total points
ID: 39218630
separate management network might help you, but remember a broadcast storm normally congests the switch cpu as well, meaning it may not be able to process things like command line input!

You can reset the password without losing the configuration by following these instructions:-
http://www.cisco.com/en/US/products/hw/switches/ps628/products_password_recovery09186a0080094184.shtml

Spanning tree really needs to be configured on *ALL* switches to be effective.
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eeRoot earned 125 total points
ID: 39220739
Usually, the switch will log which ports the loop was detected on.  Does the command "sh log" show any errors or messages about spanning tree or detected loops?
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