75 percent reversable bandwidth rule | QOS

[sarahr202]

Hi everybody

According to my book ,Cisco recommends to reserve upto 75 percent bandwidth on serial links  for CBWFQ classes.

My question is : does this CBWFQ classes  include class default as well.

Second question is why are we saving the 25 percent of link bandwidth?

Thanks.

[Joseph W. Doherty · Accepted solution]

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Posting

Pre-HQF CBWFQ, by default, would not allow you to allocate more than 75% of bandwidth to non-class-default classes.  HQF CBWFQ no longer has same restriction.

The thinking was (pre-HQF) you may have forgotten to allocate some bandwidth for "control" traffic, so the default configuration tried to avoid this becoming an issue.  The thinking is (HQF), we're all QoS adults, and you're supposed to know what you're doing. 

CBWFQ always has a class-default.  You may define it explicitly, if not, CBWFQ still has such a class implicitly.

BTW, even when CBWFQ, by default, didn't allow you to explicitly define more than 75% bandwidth, the 25% was still usable but other classes.  CBWFQ bandwidth allocations set a guaranteed minimum bandwidth when there's congestion, but unused bandwidth allocations are available to other class's traffic.

[sarahr202]

BTW, even when CBWFQ, by default, didn't allow you to explicitly define more than 75% bandwidth, the 25% was still usable but other classes.  CBWFQ bandwidth allocations set a guaranteed minimum bandwidth when there's congestion, but unused bandwidth allocations are available to other class's traffic.

Let say we have 75 percent rule applied.  We have 100 mig circuit, we are using 75 mig to allocate to our classes:

class1

class2

class3

All these classes are fully congested and all collectively using 75 mig,  we still have 25 mig unused bandwidth,  Will this 25 mig  be used by Class default only or will this 25 mig be shared among class1,class2,class3 and default class  proportionally to their bandwidth allocations?

thanks

[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 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

If class-default isn't using some of its bandwidth, that unused bandwidth is available to other classes.  Bandwidth is shared proportional.  (This is true for any class bandwidth.)

If it helps, what's actually happening, when you assign bandwidth to a class queue, that queue's traffic obtains a "weight".  When there's congestion, packets are dequeued in proportion to their weights.

Forgetting class-default, suppose you have 3 classes like:

class A

bandwidth percent 25

class B

bandwidth percent 25

class C

bandwidth percent 50

You would get same scheduling ratios, i.e. same QoS, (although not same weigths) for:

class A

bandwidth percent 1

class B

bandwidth percent 1

class C

bandwidth percent 2

If all 3 classes wanted all 100% of the bandwidth, classes A and B would each get 25% and class C 50% (just like the numbers if the first set of bandwidths, but again, same results with the second sets of bandwidth numbers).

If only two classes were active, and each wanted all bandwidth, if it was classes A and B, each would get 50% (doesn't match any bandwidth percentages, but does match ratio 25:25 or 1:1).  If it was classes A and C, class A would get 1/3 and class C would get 2/3 (again doesn't match any bandwidth percentages, but also again, does match ratio 25:50 or 1:2).

[sarahr202]

thanks Joseph