QoS question about class-default traffic

Unanswered Question
Feb 3rd, 2009

Hello,

According to cisco Qos Configuration guide

"The sum of all bandwidth allocation on an interface cannot exceed 75 percent of the total available interface bandwidth. The remaining 25 percent is used for other overhead, including Layer 2 overhead, routing traffic, and best-effort traffic.Bandwidth for the CBWFQ class-default class, for instance, is taken from the remaining 25 percent".

What happens if I define the class default traffic ? e.g

policy-map qos

class a

bandwitdh percent 60

class class-default

bandwidth percent 15

Now where do the 25% go?

Is it added to the 15%?

Is it used only for layer 2 overhead and routing?

or gets distributed evenly between the classes?

I have this problem too.
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Joseph W. Doherty Tue, 02/03/2009 - 05:42

My understanding is, if you explictly define bandwidth for class-default that bandwidth is "reserved" just like any other explict bandwidth, i.e. comes from the max-reserve (default 75% - BTW: can [always?] be changed, i.e. you can exceed 75%). The set aside bandwidth (default 25%), if not otherwise being used, is available to other classes. I also believe it's not distributed evenly but in proportion to the other classes bandwidth allocations. (This is also true for any bandwidth that's not being used by other classes or that wasn't reserved to begin with.

e.g.

policy-map qos

class a

bandwidth percent 10

class class-default

bandwidth percent 5

The above only "reserves" 15%. The other 85% would be available to overhead and to both class a and class class-default. If both those classes wanted all the bandwidth, and assuming overhead wasn't using any, the remaining 85% available would be allocated 2:1, or class a would get 2/3% of the link and class class-default 1/3% of the link.

Functionally the following policies, I believe, would behave very much alike.

Yours:

policy-map qos

class a

bandwidth percent 60

class class-default

bandwidth percent 15

and

policy-map qos

class a

bandwidth percent 4

class class-default

bandwidth percent 1

or

policy-map qos

class a

bandwidth remaining percent 80

class class-default

bandwidth remaining percent 20

jasonviar Mon, 12/13/2010 - 09:16

Is there a command that shows the allocated bandwidth for the class-default?

"show policy-map interface xx" will show for the other classes where it has been explicitly defined, but nothing mentioned for class-default.

I am referring to a situation where the class-default is not explicitly set, and therefore is using the "best-effort" availability in the 25%.

Shelley Bhalla Mon, 12/13/2010 - 09:28

There is not command that will tell you the bandwidth it will have. Default class is a catch all class hence no reason to have a bandwidth statement under that too.

by default 75% of bandwitdth is reserve by a customer QoS policy out of which you can give up-to 75 to custom classes and rest is used for process traffic, default class etc. with HQF introduced from 12.4(20)t onwards this was increased to 99%.

Shelley

jasonviar Mon, 12/13/2010 - 09:49

OK, didn't know if there was a way to see what the BW is for the class-default. Sounds like no, other than 25% of the bandwidth statement. Thanks for the quick response.

Shelley Bhalla Mon, 12/13/2010 - 10:46

Infact now that you got me thinking
Try using the command "bandwidth remaining percent 100" and see if that will trigger a counter on the default class to show you the bandwidth.

Remember you can only use either precent or actual bandwidth in all classes (except priority).

Shelley

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