Cisco switch output drops due to QoS

[wkw.domain1]

Hi,

I’m quite new to QoS and troubleshooting an output packet drop issue on a 3750x switch which seem to be due to QoS configuration.

I was following the instruction on this document for troubleshooting.

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/116089-technote-switches-output-drops-qos-00.html

As per the document, I can try increasing the buffer size of the 2^nd queue to see whether it fix the drop issue.

However, the question I have is about the concept between the traffic sharing and shaping. The document says that the sharing allows a queue to use idle bandwidth of other queues if required.

How can I find out shaping and sharing policies configured on the switch?

According to the second diagram, shaping is applied to the 4^th queue while other queues use the sharing.

[Mark Malone]

Hi

For egress I think queue 1 below or 0 in your statistics output should be the shaped and the other 3 shared , whats your global mls output that usually shows something related to queues sharing shaping , with 3750s its difficult with mls qos to get rid of output drops because the buffer is so small and its split across 4 queues as soon as mls is enabled with a decent load your likely to see some , have had to tweak the buffers a few times but you need to be careful not to cause further issues

Feature	Queue 1	Queue 2	Queue 3	Queue 4
Buffer allocation	25 percent	25 percent	25 percent	25 percent
WTD drop threshold 1	100 percent	200 percent	100 percent	100 percent
WTD drop threshold 2	100 percent	200 percent	100 percent	100 percent
Reserved threshold	50 percent	50 percent	50 percent	50 percent
Maximum threshold	400 percent	400 percent	400 percent	400 percent
SRR shaped weights (absolute) 5	25	0	0	0
SRR shared weights 6	25	25	25	25

[wkw.domain1]

Hi Mark and Joseph,

Thanks both for the responses and they cleared some of the concerns I had.

As per the interface stats, the drops are noticed on the queue 2 ( if you count from 1 ).

In theory, this queue should be able to have 100 % of the bandwidth if the link is idle. Is this correct ?

If that's correct, I am not clear why the packets are getting dropped instead of using the full bandwidth. As per the packet capture I took, there is not much traffic going through the interface so the link is almost idle apart from occasional spike of data 100 kb lasting few milliseconds.

I think that the packet drops are expected if we assume this queue is shaped based on my following calculation.

link bandwidth = 100 Mbps

bandwidth per queue = 25 Mbps

bandwidth per queue per millisecond = 25 kb

So, some packets can get dropped if there are 100 kb traffic per millisecond.

Do you think this is correct or am I talking nonsense ?

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

In theory, this queue should be able to have 100 % of the bandwidth if the link is idle. Is this correct ?

If the queue or port isn't being shaped, yes you should be able to use 100% of the bandwidth. However, microbursts can cause drops that are hard to see with typical interface utilization stats.

[Mark Malone]

If its is micro burst which it is quite often from some application this may help

https://notalwaysthenetwork.com/2014/01/06/microburst-detection-with-wireshark/

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Nice reference!

However I would debate the reference's "If you are consistently seeing output drops increment in combination with reaching the line speed of the interface your best option is to look into increasing the speed of that link."

If you're using TCP, and you have applications that congest the link, drops can be a natural part of TCP flow control.  Accepting drops might be perfectly acceptable, as long as the application service needs are met.  Additionally, QoS, or TCP tuning (to optimize BDP or use a newer/smarter TCP stack) may minimize or eliminate drops.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

How can I find out shaping and sharing policies configured on the switch?

show mls qos int <interface> <interface number> queueing

PS:

I’m quite new to QoS and troubleshooting an output packet drop issue on a 3750x switch which seem to be due to QoS configuration.

Quite often the case with the 3560/3750 series, due to both their small buffers and their default QoS settings.

I've often found the best solution is moving interface reserved buffers to the common pool and then increasing the tail drop settings.  With such a configuration I've seen interfaces that have multiple drops per second decrease to a few drops per day.