[c-nsp] C2960G and output drops

Brad Henshaw brad.henshaw at qcn.com.au
Wed Oct 1 20:55:46 EDT 2008

Gert Doering wrote: 

>     Queue 3
>       Weight 0 Frames 0
>       Weight 1 Frames 0
>       Weight 2 Frames 3505735486
> ... so that basically tells me what I was assuming - "all packets go
> into the same queue (no QoS marking going on here, all packets are
> important), and some of them get dropped".
> I'm not really sure how to start from here towards "*why* is it
dropping the packets"?

I've gone through this grief before and microbursts + insufficient
default buffer drop thresholds were the theory of the day - adjusting
the thresholds resolved the problem after TAC spent many weeks looking
at the issue - but in our case, we /did/ have mls qos enabled. I have no
idea how these switches manage buffer allocations when mls qos is

Existing queue-set parameters can be checked with 'show mls qos
queue-set'. The buffers and drop threshold can be adjusted with 'mls qos
queue-set output buffers' and 'mls qos queue-set output threshold'.
Obviously it's quite possible these won't do anything unless mls qos is
enabled so a workaround might be to fiddle with these in the lab (with
mls qos enabled and disabled) to see if you can reproduce and resolve
the problem. We had to increase the drop thresholds for the affected
queue from 50 to 400.

Note the queue numbers in these commands are offset by one compared to
the 'show mls qos port-asic stat' output.

Then again maybe it's not microbursts and is something else entirely.
Have fun with the TAC!


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