3750 Total output drops

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Jan 21st, 2010
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I have a 3750 switch that has a bunch of workstations plugged into it, I'm having an issue where alot of the ports have a good amount of output drops and I can't seem to figure out why.  All the workstations are on 1Gb links and are very underutilized at about 6Mbs, so I know the drops aren't caused by over utilization.  Under show int counters errors every dropped packet falls into "outdiscard".  Is there anyway for me to capture just the packets that are being dropped?  Any idea what would be causing this?  Here is an example from one of the ports:

sho int

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 183339
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 191000 bits/sec, 145 packets/sec
5 minute output rate 5903000 bits/sec, 1506 packets/sec

sh int counters errors:

Port        Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards
Gi1/0/24            0           0           0           0          0       183339

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Itzcoatl Espinosa Thu, 01/21/2010 - 08:01
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When troubleshooting output drops on an interface I suggest the following:

a) Change the load interval from 5 minute to 30 seconds.

b) Clear the counters.

c) Check continously the output queue, to verify how fast the drop counters increase.

If you are not overutilizing the circuit, maybe you just are running applications that send traffic bursts. This could be translated into output drops as the interface queue is not able to handle these peaks of traffic. If that is the case, you could try to increase the output queue (there is no recommended value), but it might help a little bit.

But if you have bursty traffic there is no much thing to do.


Itzcoatl Espinosa


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