ONT bandwidth remaining percent

Unanswered Question
Apr 2nd, 2008

I am having a hard time understanding how the remaining percent bandwidth calculations take place.

From an answer in the 642-845 ONT book CD I am told the following.

The Bulk class has been allocated 20% of the remaining bandwidth, after allocating 20% to the Priority queue, 5% to Signaling and 40% to Transactional. 20% of the remaining 26Mbs is 5Mbs.

This is a FastEthernet interface, so it is 100Mbs with 75% available by default.

I calculate the bandwidth as follows:

Queue___________%_____Avail Mb___Allocated





Can someone explain to me what I am missing. There are no solid examples of this in the book using real numbers. Obviously 26 != 34 and 7 != 5.

I have this problem too.
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wagnerm Mon, 04/14/2008 - 11:27

Is this the CCNP ONT - official exam cert guide by Amir Ranjbar. If this is the same book, what chapter is your question from? I have this book and couldn't find it.

doseyg Mon, 04/14/2008 - 15:32

Yes, this is that book. The question came off of the included CD, but upon review it does not seem to be included in the book, so it must be one of the practice test questions on the CD. In my case the example configuration for the question is contained in the file 1587201763-Q170.txt

wagnerm Tue, 04/15/2008 - 04:58

I found it. I got the same answer as you did. I believe that this is another question with an incorrect answer. I just started the first 2 chapters and when I choose the book questions on the CD, I find 2 errors. Question ID2 and ID29 are not right. However, if you look in Appendix A, the book has the correct answer.

You figured out the correct answer for the question, the CD is wrong.

mheusing Tue, 04/15/2008 - 05:26


the calculation is not entirely correct. First, you can either only use bandwidth remaining percent in all CBWFQ classes or bandwidth percent or bandwidth .

With bandwidth remaining percent the calculation would look like this:


Priority:_______20%_____20 Mbps !!Note priority percent calculates against interface bandwidth !!

Remaining BW: 55 Mbps (=100*0.75 - 20)

Signaling:______remaining 5%_________3 Mbps

Transactional:__remaining 40%________22 Mbps

Bulk:___________remaining 20%________11 Mbps !!Note "bandwidth remaining percent" calculates with remaining BW of 55 MB and not with available BW!!

Result: Available BW = 100*0.75 - 20 - 3 - 22 - 11 = 19 Mbps

In case you configure Signaling and Transactional with priority queues (not recommended):


Priority:_______20%_____20 Mbps !!Note "priority percent" calculates against interface bandwidth !!

Signaling:______5%_________5 Mbps

Transactional:__40%________40 Mbps

Available BW: 10 Mbps (100*0.75 - 20 - 5 - 40)

Bulk:__________remaining 20%________2 Mbps !!Note "bandwidth remaining percent" calculates with Available BW of 10 Mbps !!

In brief:

"priority percent" and "bandwidth percent" refer to interface bandwidth

"bandwidth remaining percent" refers to remaining bandwidth = interface BW x max-reserved - Sum of all priority reservations

Hope this helps! Please use the rating system.

Regards, Martin

doseyg Tue, 04/15/2008 - 07:56

Thanks Martin,

Your explanation is very clear and explains in several sentences what several pages of the book failed to effectively convey to me. I could never understand why you would want a router to calculate the bandwidth how I was doing it. What you've explained makes so much more sense.

Also, thanks for clarifying that priority bandwidth is calculated against the interface but subtracted from the unreserved.

For anyone else who happens to read this, here is the configuration we are discussing. I apologize, I should have included this to start with.


policy-map AutoQoS-Policy

class AutoQoS-Voice

priority percent 20

set dscp ef

class AutoQoS-Signaling

bandwidth remaining percent 5

set dscp cs3

class AutoQoS-Transactional

bandwidth remaining percent 40

random-detect dscp-based

set dscp af21

class AutoQoS-Bulk

bandwidth remaining percent 20

random-detect dscp-based

set dscp af11

class AutoQoS-Scavenger

bandwidth remaining percent 1

set dscp cs1

class class-default



muca Tue, 08/11/2009 - 20:57

How can you get a remaining bandwidth of 55 if it based on percentage?

Am I missing something? Shouldn't it be 60?

Available is bandwidth 75000kbps

priority percent 20 = 15000bps

75000-15000 = 60000 so the bulk traffic gets 12mpbs

mnorwood Fri, 08/21/2009 - 11:42

See Martin's comments earlier in the thread.

He states:

"Priority:_______20%_____20 Mbps !!Note priority percent calculates against interface bandwidth !! "

55 is correct if you calculate as follows:

20% of 100Mbps = 20Mbps

Avail bandwidth = 75Mbps

75Mbps - 20Mbps = 55Mbps

sharor332 Sun, 09/05/2010 - 22:55

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So if in the scenario below, we have a 1Gig interface with interface bandwidth of  1G with a

         max-reserved-bandwidth 100 configured

         A nested service-policy in use.

Nested service-policy details

         Parent policy shapes to 400Mbps.

         Child policy has a VOICE-class with strict priority percent 20 example below

Policy-map parent-testing

  class class-default  shape average 4000000 60000 0

policy-map TESTING

class VOICE

    priority percent 20

class class3

    bandwidth remaining percent 40

class class2

    bandwidth remaining percent 30

class class1

    bandwidth remaining percent 10

class default

- Can you clarify my understanding on the behavior of this config -

1. The voice-class 20% is calculated off the parent policy – hence voice-class gets a max 20% of 400M =  80M strict priority. Policed at 80M during congestions.

2. When there is no congestion on the link– that is say Voice-class, class3, class1, default-class have NO packets in it; can Class2 scavenge upto 400Mbps ??  OR can Class2 go upto 320Mbps only (even though there is nothing in voice class)

3. During periods of high congestion, when all classes are congested to the full. Can you confirm when using “bandwidth remaining percent ” command, as per the config- if –

     a.Class3 will get a guaranteed minimum 128Mbps (40% of 320Mbps)

     b.Class2 will get a guaranteed minimum of  57.6 Mbps (30% of remaining [400-80-128]) OR

     c.Class2 will get a guaranteed minimum of 96 Mbps (30%of 320Mbps)

     d.Class1 will get a guaranteed minimum of 13.44Mbps (10% of remaining [400-80-128-57.6]) OR

     e.Class1 will get a guaranteed minimum of 32Mbps (10% of 320Mbps)

4. Where does the priority percent calculates against interface bandwidth In this case?? should the calculations not be based on the parent policy map in this case ?




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