Problem tracking down port usage.

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May 12th, 2010
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Hi All,

I had a problem today with a Catalyst 3750 (100mb Version) that is connected via a single uplink to a stack of Gigabit 3750's, the uplink is a trunk (802.1q) default settings, no vlan pruning, both switches are in the same VTP Domain and the management interface is in a common vlan.

All devices on the 100Mb 3750 we're either loosing packets or reporting response times of >50ms, upon examination of the uplink between the switches I found the below:

5 minute input rate 98508000 bits/sec, 8941 packets/sec (Based on my calculations this is 92Mb/11.5MB/s which would saturate a 100Mb link).
5 minute output rate 63000 bits/sec, 47 packets/sec

That's the output from the 100Mb 3750 side of the link the output from the Gigabit Core (Stack) is as below:

5 minute input rate 60000 bits/sec, 60 packets/sec
5 minute output rate 98093000 bits/sec, 8888 packets/sec

My question is how in future can I find out easily what's generating the bandwidth? I went through every interface on the 100Mb 3750 and the 5 minute input/output rate for all interfaces was nothing like that of what was flowing through the uplink.

Is it just a case of getting some network monitoring software to monitor input/output of each interface or is there a smart way of tracking down the heavy users/generators of traffic?

Obviously with the trunk carrying all Vlan's I suppose it's possible that a machine in a given Vlan was generating a large amount of broadcast traffic that was going up/down the link.

I presume pruning unused Vlan's from the link would be a prudent idea going foward to recude uncessary traffic to that switch? Manual or Automatic pruning?

Kind Regards,


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glen.grant Wed, 05/12/2010 - 07:40
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  An easy way to look at the per port utilization % on a 3750 is to use the "show controllers utilization " command which will give per port utilization in a easy to look at format  and you can see if any 1 port may be driving that utilization .

DuncanM2008 Wed, 05/12/2010 - 08:10
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Hi Glen,

That sounds like just want I need for getting at a glance heavy users per-port, trouble is on the 3750's we're running the command isn't available.

sh controllers is but utilisation is not an option/sub-option.

I presume it was implemented in a later IOS release?

Currently running: C3750-i9-mz.122-20.SE4.bin



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