bandwidth remaining percent calc.. need clarity

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Sep 6th, 2010
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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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james.mirtsis Sun, 09/12/2010 - 19:10
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Hi there,

             it is my understanding that:

1. voice traffic would get all 400mbps until there is congestion at which point it is then policed to 80mbps.

2. can scavange up to 400mbps. qos, policing and shaping only come into play when there is congestion not when there isn't.

3a. class 3 will get 40% of 320mbps.

  b. class 2 will get a guaranteed rate of 30% of 320mbps.

  c. no class 2 will get as per b.

  d. correct

  e. incorrect, correct is d.

4. it can only take the bandwidth allocated by the parent policy not the actual interface bandwidth, so in this case to 400mbps. 


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