High interface discard rates - What causes it?

A colleague has asked a question about some discard rates he is seeing on one or 2 switchports.  they have surpassed a threshold limit of 10%.

Can somebody tell me what are all the possible causes of high discard rates on an interface?

Thanks

mac326aAsked:
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DCenaculoConnect With a Mentor Commented:
High discard rates may indicate that the device cannot keep up with traffic. Use Top broadcasts or L2 multicast flows with Units selector set to Frames/s to help identify sources of high broadcast or multicast traffic.
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DCenaculoConnect With a Mentor Commented:
CISCO

High Discard Rate
This fault is reported if:

"The input packet queued rate is greater than the minimum packet rate, and the input packet discard percentage is greater than the Discard Threshold. The input packet queued rate is the rate of packets received without error. The input packet discard percentage is calculated by dividing the rate of input packets discarded by the rate of packets received.

"The output packet queued rate is greater than the minimum packet rate, and the output packet discard percentage is greater than the Discard Threshold. The output packet queued rate is the rate of packets sent without error. The output packet discard percentage is calculated by dividing the rate of output packets discarded by the rate of packets sent.
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DCenaculoConnect With a Mentor Commented:
Deferred Frames (Out-Lost or Out-Discard)
If you have a large number of deferred frames, or Out-Discard (also referred to as Out-Lost on some platforms), it means that the switch's output buffers have filled up and the switch had to drop these packets. This can be a sign that this segment is run at an inferior speed and/or duplex, or there is too much traffic that goes through this port.

For CatOS, use the show mac command for the module and port or the entire module to look at out-discards:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml#outlost
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Aaron StreetConnect With a Mentor Infrastructure ManagerCommented:
i was going to say make sure speed and duplex match first of all..

this can deffently cause this problem.

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