QoS Default-Class Drops

jsmcniel88 · ‎07-29-2012

We are seeing a lot of drops on our default class even though we are not even half way to our shape limit. Looked around and just cant nail down a strait answer for the drop, any thoughts?

Class-map: class-default (match-any)

117573391 packets, 97334962774 bytes

5 minute offered rate 14886000 bps, drop rate 100000 bps

Match: any

Queueing

queue limit 64 packets

(queue depth/total drops/no-buffer drops/flowdrops) 0/52661/0/0

(pkts output/bytes output) 117520733/97241082592

Fair-queue: per-flow queue limit 16

Exp-weight-constant: 9 (1/512)

Mean queue depth: 0 packets

class class-default

fair-queue

random-detect dscp-based

policy-map PARENT

class class-default

shape average 45000000 3000000 1000

service-policy WAN-EDGE

Thanks

nkarpysh · ‎07-29-2012

Hello,

It looks that traffic going through class-default is bursty.

Your command "shape average 45000000 3000000 1000" telles the router/switch to split each second interval into 15 time slots (TC 45/3). During each time slot you can send not more than 3000000 bits. So you can get e.g. 6000000 bits in single time slot and you wont be able to send that all - and your extended burst is very small to allow you to send that all - thus you will drop it.

Thus you can get 6000000 per second which is lower than your CIR but if that came during single TC - then you should indeed expect drops.

You can tune your settings or use Cisco recomended values for Bc and Be:

normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds

#

extended burst = 2 * normal burst

You can go through the links below to understand it in more depth

http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/polcing_shping_oview.html

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfpolsh.html#wp1011592

HTH

*You can rate if it was useful*

Nik

HTH,
Niko

jmcgrady1 · ‎06-27-2022

I am seeing a similar issue. Not a lot of traffic is matching my class maps. Most is hitting the class-default. There, 200kbps is dropped from the 27mbps total.

Is there a command which will show me what traffic is hitting the class-default and, even better, what kind of traffic is being dropped? If so, i can factor it into some specific class maps

Joseph W. Doherty · ‎07-30-2012

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Posting

Your class-default's (default) queue depth is possibly too shallow for 45 Mbps. I would recommend sizing it at about half your BDP (bandwidth delay product).

jsmcniel88 · ‎07-30-2012

Correct me if I am wrong but that is achieved by useing the queue-limit X command for class class-default?

Joseph W. Doherty · ‎07-30-2012

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 whatsoever (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

Correct.