qestion about QoS

Unanswered Question
Sep 21st, 2008
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following is the question from CCNP ONT (ISBN-13: 978-1-58720-176-9). I do not know how to calaculate it. Below is the question and answer.

Does the book give the wrong answer?

----- Question ---------

Examine the output shown, base on this configration, how much bandwidth would be allocated to FTP traffic if the policy was applied to inter Fa1/0/0?

A. 8M

B. 20M

c. 1M

d. 5M

e. 16M

f. 40M

------- Configration: --------


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



------ Answer -------

20% of 26M is 5M, the ans is "d"

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Overall Rating: 4 (2 ratings)
Giuseppe Larosa Sun, 09/21/2008 - 02:07
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Hello Anita,

here you have two concepts:

the max-resevered bandwidth that is 75% of the link speed, 25% is left for routing protocols.

of this 75% available to user traffic:

20% is used by LLQ : priority percent 20

the bandwidth remaining percent operates on what is left:

100-25-15 = 60

so the resurces for transactional class are:

40% of 60 Mbps = 24 Mbps

for bulk:

20% of 60 Mbps = 12 Mbps

for signalling:

5% of 60 Mbps = 3 Mbps

I don't agree too with the book answers

Hope to help


anitachoi3 Sun, 09/21/2008 - 04:18
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I do not know the "meaning" of "remaining percent". Does it mean that I must use the "remain b/w" for the next calculation?


75M x 20% = 15M

60M x 5% = 3M

57M x 40% = 22.8M

34.sM x 20% = 6.8M


Marwan ALshawi Sun, 09/21/2008 - 04:28
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i think this calculation depends on the typ of traffic the utilizing the link

for example

if the link is useing voice which is 20% and then the bulk started then the 20 persnt will be 60 x 20% then the amount will be diffrent

so it remaining depends on how much left no always sequetioly allocated as above

Giuseppe Larosa Sun, 09/21/2008 - 04:35
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Hello Anita,

remaining percent references to the bandwidth that is avaliable after that:

you subtract the bandwidth left to routing protocol (25% of link speed / default bandwidth settings)

and you subtract the bandwidth reserved with the priority command for other traffic class.

So in the question scenario:

100 Mbps = link speed = default bandwidth

75% of 100 = 75 Mbps

of this 20% is used br priority queue

so remaining percent should refer to:

100-25-15 = 60 Mbps

the bandwidth interface command can be used to change the reference BW for these calculations.


int f1/0

bandwidth 30000

sets the BW to 30 Mbps

Hope to help


Joseph W. Doherty Sun, 09/21/2008 - 11:49
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Like the other posters, I don't believe the answer is correct, not does the question provide sufficient information. Further, on most platforms, I believe, FQ within class-default makes the answer almost impossible to know since actual bandwidth for defined classes appears to be impacted by the number of FQ flows on those platforms.

For the most part, ignoring the above, we can try to calculate the minimal bandwidth allowance for FTP as follows:

First, we need to set aside bandwidth reserved for non-user defined classes. The default is 25%. (NB: The default can be changed.)

Assuming the fast Ethernet is 100 Mbps, this leaves us with 75 Mbps. (The rest works, I think, as follows.)

The LLQ classes, only one defined in the question, allocate 20% of the 100 Mbps (20 Mbps) which leaves us with 55 Mbps of "remaining" bandwidth.

Assuming FTP maps into the Bulk class, bandwidth remaining percent 20 (of the 55), should reserve a minimal of 11 Mbps but the sum of the percentages of all the other defined classes only total 66% remaining, not 100%. So, where does that other 33% (18.15 Mbps) of 55 Mbps go?

I've understood undefined bandwidth is proportioned the same as the classes proportion their bandwidth but there's Cisco documentation that also notes the undefined bandwidth is equally distributed to all the classes. So, with the latter, Bulk gets its 11 plus another 4.5 (1/4 of 18.15), about 16 Mbps. With the former, Bulk again gets its 11 plus antoher 5.5 (20/66% of 18.15). All this assumes all defined classes are demanding all possible bandwidth and FQ within class-default is only limited to 25% of the 100 Mbps (which, again, don't believe it's true for most platforms).


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