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Switch loop suggestion - Cisco Router

Posted on 2012-03-14
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Last Modified: 2012-03-15
Hello Experts,

We used to have the following setup in a "cascade" setup.
3comA-connects to 3comB- 3comC connects to 3comD to 3comE and so fort. Total 7 Switches.

We bought a Cisco 2960 and decided to have all the switches connect to the Cisco as the
"Backbone Switch".

So we broke the "cascade" and now every switch connects to a different port on the Cisco 2960.

We are getting a loop after  this setup, every now and then different parts of the network loose connectivity.  We only have Vlan1 enabled and the default setup that came with the Cisco Switch.

To avoid any other switch from becoming a "root bridge", I ran the following command on the Cisco Switch so that it could be the main router:  
spanning-tree vlan 1-4094 priority 0.   (This did not work)

Any ideas on how I can find the loop?

Thanks,
R
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Question by:RandallVillalobos
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by:
eeRoot earned 400 total points
ID: 37723437
Are the 3com switches still attached to each other, or only the 2960?  If you type "sh log" on the 2960, are there any errors about ports being disabled.

Can you post a wiring diagram that shows the new network layout, the running config from the 2960, and the results of the "sh log" command?
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Assisted Solution

by:mat1458
mat1458 earned 100 total points
ID: 37725143
Are you sure that the 3com switches have spanning-tree enabled? What type of switches are they?

I'd look for mac-flap messages in the log. These messages would show you on which ports mac addresses move from one to another. Then you can trace the source of the loop.
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Author Comment

by:RandallVillalobos
ID: 37726773
I see the following on the show log:

*Mar  1 06:13:14.327: %IP-4-DUPADDR: Duplicate address 192.168.1.2 on Vlan1, sourced by e83e.b62d.03d9
*Mar  1 06:15:43.770: %IP-4-DUPADDR: Duplicate address 192.168.1.2 on Vlan1, sourced by e83e.b62d.03d9
*Mar  1 06:26:06.549: %IP-4-DUPADDR: Duplicate address 192.168.1.2 on Vlan1, sourced by e83e.b62d.03d9
*Mar  1 06:26:53.827: %IP-4-DUPADDR: Duplicate address 192.168.1.2 on Vlan1, sourced by e83e.b62d.03d9

Looks like a duplicate ip could be causing the loop. We have modified the management ip address.

How can I clear / erase the Log and review the results after changing the ip address?

I don't have access to the config but I will upload as soon as it is available.
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LVL 22

Expert Comment

by:eeRoot
ID: 37727672
The command to clear the log is either "clear logging buffer" or "clear logging"  You have to be in config mode to use it, like this:

conf t
clear logging
end
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Author Closing Comment

by:RandallVillalobos
ID: 37727846
Thank you!
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