Temporary High Discard Rate...

Unanswered Question
Apr 23rd, 2010
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Hello,


I have an interface on a 4948-48port that temporarily snmp's some high discard rate events, that always are short lived. I am going to open a case I think but I thought I would ask the question here what might be going on. Any help would be appreciated.


EVENT DESCRIPTION = HighDiscardRate::Component=PORT-xxx.xxx.xxx.xx/0.26 [Gi0/2];Type=ETHERNETCSMACD;InputPacketRate=1.7895722E7 PPS;MinimumDiscardRate=8333.328;DuplexMode=FULLDUPLEX;OutputPacketRate=1.4958334 PPS;DiscardThreshold=5;InputPacketDiscardPct=99.99986 %;InputPacketDiscardRate=1.7895698E7 PPS;MaxSpeed=1000000000;OutputPacketDiscardRate=0.0 PPS;OutputPacketDiscardPct=0.0 %;

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By temporary I take it that you mean it only occurs for a short period of time then stops.



There can be several reasons for this, but the most common reason is ipv6.  If you have introduced windows 2008, Vista, Win7, or enabled ipv6 on another system then you will get temporary spikes during the day via ipv6 classless broadcasts.


There are other reasons you could get this, but the easiest way is check the logs, run packet sniffers, and all the other stuff you would normally do for high discard rates.  The only additional difficulty you have is that you need to respond fast enough to start a trace, do something that will run all the time, or think of a way to kick off a job to run the packet trace.


If you are already dumping your logs onto a linux/unix server then you can run a cron/sql job every minute on the logs and have it kick off the packet sniffer when it detects the discards.



(edit)

it is probabally best that I say can and not will for ipv6 classless broadcast causing discards.  That really depends on your settings, but by default they usually do not have the proper settings to handle ipv6 packet sizes.



(edit2)

I should of elaborated more on my assumption:

By temporary I take it that you mean it occurs for a short period of time then stops, but then resumes again later.  Not temporary as has only happened a small handful of times.

Ganesh Hariharan Mon, 05/03/2010 - 22:28
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Hello,


I have an interface on a 4948-48port that temporarily snmp's some high discard rate events, that always are short lived. I am going to open a case I think but I thought I would ask the question here what might be going on. Any help would be appreciated.


EVENT DESCRIPTION = HighDiscardRate::Component=PORT-xxx.xxx.xxx.xx/0.26 [Gi0/2];Type=ETHERNETCSMACD;InputPacketRate=1.7895722E7 PPS;MinimumDiscardRate=8333.328;DuplexMode=FULLDUPLEX;OutputPacketRate=1.4958334 PPS;DiscardThreshold=5;InputPacketDiscardPct=99.99986 %;InputPacketDiscardRate=1.7895698E7 PPS;MaxSpeed=1000000000;OutputPacketDiscardRate=0.0 PPS;OutputPacketDiscardPct=0.0 %



Hi ,


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 and other cause for high discard rate can be "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.


Check out the below link for troubleshooting switch ports


http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml#outlost


Hope to Help !!


Ganesh.H


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