3560x Output Drops

[Daniel Smith]

I have a monitor session process set up to forward data from 4 ports to a single output port. I am surprised to see that on the output port, the rate of output drops is steadily increasing:

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 153449

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  30 second input rate 0 bits/sec, 0 packets/sec

  30 second output rate 59,733,000 bits/sec, 7698 packets/sec       <I added the commas, nice view I think.

The output drops are being blamed for another applications failure to adequately monitor the combined output of the four ports. I do not see how at 60Mbps, this interface should be dropping so much traffic. THanks in advance for any suggestions!

[Sandeep Choudhary]

Check it out:

https://supportforums.cisco.com/community/netpro/network-infrastructure/switching/blog/2011/04/11/output-drops-due-to-qos-on-296035603750-switches

Reagrds

[Joseph W. Doherty]

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Posting

A common issue with the 3560/3750 series, is "shallow" buffers.  I.e. if there are bursts of traffic that exceed the ports transmission rate, the port will drop packets even though the "average" transmission rate isn't high.

Mapping 4 ports to 1 port could create such a situation.  Although you have your load average set to 30 seconds, a lot can happen within 30 seconds.

[Leo Laohoo]

"Total output drops" is not a "network issue", per se.

This counter increments when the downstream clients starts dropping packets.  This happens when the downstream packet can't process the volume of incoming traffic.