QoS- bandwidth remaining percent.

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Sep 9th, 2010
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Much appreciate if someone can shed some light –

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)


2. 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)


3. 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 ?


Thanks.

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Lei Tian Thu, 09/09/2010 - 20:28
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Hi,


 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)


2. 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)


3. 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 ?

1.yes


2. Class2 can use upto 400Mbps when there is no congestion.


2. I think the remaining is 'full bandwidth - PQ reserved bandwidth', so, a,c,e.


3. The priority is calculated against parent level shapper rate; do you see you got 1G*20% on your case?


Regards,

Lei Tian

ashok_boin Fri, 09/10/2010 - 02:27
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Hi Sharor,


The "bandwidth remaining percent" command  is an interesting command which does not give absolute percentage  values rather it provides relative percentage values allocation. Answers to your questions inline.



-------


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. CORRECT.

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) Yes, it can.

2.  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) Correct

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) Correct

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) Correct

3.  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 ? Parent policy map shaper rate (in this case, 400M)


---------


Here is an interesting example.


If Voice class is using 60Mbps, then the remaining bandwidth = 400M-60M =340M. Then, other classes will get relative bandwidth out of this free bandwidth like Class 3 will get 40% of 340Mbps = 136Mbps.


Please go through the following link for detailed explanation.


http://www.cisco.com/en/US/docs/ios/12_2t/12_2t2/feature/guide/ftLLQpct.html#wp1026343


Regards...

-Ashok.

siddhartham Wed, 01/16/2013 - 11:28
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Thanks for the explananion.


Siddhartha

Joseph W. Doherty Wed, 01/16/2013 - 18:50
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To start with, I'm unsure what your non-LLQ bandwidth allocations are as you didn't define bandwidth for class-default nor did you specify which IOS version you're using.  (NB: there's some differences between pre-HQF and HQF CBWFQ.)


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.


Correct.


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)


No, not congestion on the link, congestion due to the shaper.

With your posted configuration, any of the non-LLQ classes can use the full bandwidth allowed by the shaper, 400 Mbps.  (And as you described in your 1st question, so can LLQ if there's no congestion.  I.e. its implicit policer only triggers when there's packets queued to the LLQ.)


2.  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)


Unsure because what class-default's percentage is undefined in your posting.

Bandwidth percentages are a bit misleading, because they are only "correct" when you allocate all 100%.  Otherwise they effectively set ratios between classes.  For example, ignoring class-default (i.e. zero traffic) and assuming all your non-class-default classes were congested, your LLQ gets 20%.  Your other class ratios are 4:3:1 or 4/8%, 3/8% and 1/8% of the remaining 80%, or .8*4/8%, .8*3/8% and .8*1/8% of your 400 Mbps.


3.  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 ?


As your priority class is in the child policy, its priority % is against the bandwidth it believes it has, in this case the 400 Mbps.  If you also defined a priority class in the parent policy, by default, it would takes its percentage from 1 Gbps.  (Some or all IOS versions, I recall, [for the interface policy classes, not child policy classes] will base their percentage against the interface's bandwidth statement.)

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