constant High CPU on 3550 and 3750 switches due to vlan 1

Unanswered Question
Jul 1st, 2009

We are experiencing constant High CPU on 3750 switches due to vlan 1

Further info - we have a pair of Alteon load balances between the 3500 and 3750's which is also running vlan 1. Could this produce high CPU ? (Appreciate vlan 1 usage not best practice but ignore that for now). We run sho proc cpu sorted and see vlan manager proc running cpu at 90% constantly. How would vlan ID manger process push cpu to 90% and hold it there ? - appreciate any ideas on this.

I have this problem too.
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Jerry Ye Wed, 07/01/2009 - 06:28

Hi David,

The problem you are discribing sound like a spanning tree loop.

Just a question, how is the switch port configuration for the Alteon? If they have spanning portfast enabled, turn it off and make the switch to run its spanning-tree algorithm to see this will improve. Also, you should schedule a maintenance window to test this configuration.



Leo Laohoo Wed, 07/01/2009 - 14:48

I agree with Jerry. Had experience with this and most of the time it boils down to spanning-tree portfast enabled to uplinks like switches.

This could be caused by someone (other than the IT department) bringing to work a number of un-managed switch or hub (oh no!) and connecting them around their respective desks.

Also, try to steer away from using VLAN 1.

Hope this helps.


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