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QoS Design Guide

Posted on 2008-06-22
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Last Modified: 2010-04-02
Hello all,

I read the White Paper, Enterprise QoS Solution Reference Network Design, and I understand the concept of classifying, marking, policing, and scheduling. However, I am still have a few questions:
1. Based on the my policy-map sample below, if you add the bandwidth percentage together, only 80% of the bandwidth will be used for scheduling. What will happen to the left-over 20% of the bandwidth? Does it mean that no traffic will be using the 20% and I am wasting the bandwidth?

policy-map wan-edge
 class marked-voice
  priority percent 25 200000
 class marked-video
  bandwidth percent 10
 class marked-callsig
  bandwidth percent 5
 class marked-critical-data
  bandwidth percent 20
 class marked-network-control
  bandwidth percent 5
 class marked-bulk-data
  bandwidth percent 4
 class marked-scavenger
  bandwidth percent 1
 class class-default
  bandwidth percent 10

2. In the white paper, Cisco talked about QoS for voice, video, and data. What if I only have critical data to prioritize? Can I have only critical data class (35% of total bandwidth) and default class (65% of total bandwidth)?

3. If I enable QoS with the scheduling in question 1 and my only traffic at that moment is the default class traffic. Will QoS allows me to use more than 10% of the bandwidth (up to 65% as the traffic cannot use the 20% of the priority bandwidth?

Thanks
NetDoc01
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Question by:netdoc01
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donmanrobb earned 480 total points
ID: 21843544
1. 25% of traffic by default is reserved for the default queue, this prevents other traffic such as routing from starving because of QoS, this can be changed at the interface with the max-reserved-bandwidth command.

2. Cisco was only offering suggestions for what to prioritze, you can set prioritize whatever traffic you deem importent. The only real restriction is not to priority too much traffic or the effect of QoS is lost.

3. QoS is only really in affect when there is congestion on the link and if there is critical traffic being sent at the time.
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Author Comment

by:netdoc01
ID: 21846230
Ddonmanrobb,

For question 1, if congestion occurs and if I understand correctly, QoS will reserve the bandwidth that I configure in my classes. Based on my classes, I only reserve 80%. Does it mean that when congestion occurs, only 80% of the link will be used to schedule the traffic. Don't I waste the left-oever 20%? Shouldn't I correct my bandwidth reservation to 100% so that I fully utilize my link?

Thanks
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by:donmanrobb
donmanrobb earned 480 total points
ID: 21847200
In your example you may actually be wasting some bandwidth because you have marked traffic that is usually covered in the default queue such as network-control traffic for example. So in your case you can safely change the bandwidth to 100%. Though it would be a better design if you left the bulk-data and network control to the default queue.
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by:donmanrobb
donmanrobb earned 480 total points
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Did you have any more questions?
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Author Comment

by:netdoc01
ID: 21862191
Donmanrobb,

If I understand correctly, the total bandwidth of the QoS classes should always be 100%. Else the queues will not be serviced at full potential if congestion occurs. In my case, I waste 20% of the bandwidth because my total percent bandwidth for the classes is only 80%. If I don't specify the percent bandwidth for my default class, then it is default to 25%.

Do I understand correctly?

Thanks
Jacques
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by:donmanrobb
donmanrobb earned 480 total points
ID: 21862308
Actually you waste bandwidth because you are classifying traffic that is normally in the default queue so the default queue itself stays mostly empty but still reserves 20 - 25%. In any case max-reserved-bandwidth will need to be set to 100 for this to work, in fact if you tried to apply the QoS to an interface you would probably get an error because the interface is still holding 25% for the default queue and will tell you you don't have the bandwidth.
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by:RTh0037
RTh0037 earned 20 total points
ID: 21862653
hey netdoc01 kind of unrelated but do you have the url for the Enterprise QoS Solution Reference Network Design white paper that you read
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by:donmanrobb
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by:netdoc01
ID: 21865255
donmanrobb,

" In any case max-reserved-bandwidth will need to be set to 100 for this to work". From your statement, if you set max-reserved-bandwidth to 100, it will override my 10% bandwitdh for my default class and make it the default (which is 25%). Is that correct?
Thx
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by:donmanrobb
donmanrobb earned 480 total points
ID: 21865456
Actually the max-reserved-bandwidth to 100 will allow your 10% bandwidth statement to work. It's not not making the default class 25%, in fact its making it 0% reserved.

As your config is now, the interface is reserving 25% (your bandwidth command under the class does nothing) Since your classifing 80% or so with other traffic this exceeds 100% and will fail. Changing to max reserved 100 disables the interface from trying to reserve 25% and the router will listen to your bandwidth statement under class-default. You could also for exampe, change the max-reserved-bandwidth to say 90 which would allow 10% for the default queue.
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Author Comment

by:netdoc01
ID: 21901216
donmanrobb,

The problem that I am having is if I have a T1 link and what if I reserve only 1268kbps, what will happen to my 276kbps? Is it not in the default class because I already reserved 23%. My total reserved bandwidth for my classes in percentage is 75%. So Please see below:

someword02-19#sh policy-map interface ATM2/0.2
 ATM2/0.34: VC 3/148 -

  Service-policy output: wan-edge

    Class-map: marked-bulk-data (match-any)
     ...
      Queueing
        Output Queue: Conversation 73
        Bandwidth 4 (%)
        Bandwidth 67 (kbps) Max Threshold 64 (packets)
       ...

    Class-map: marked-scavenger (match-any)
      ...
      Queueing
        Output Queue: Conversation 74
        Bandwidth 1 (%)
        Bandwidth 16 (kbps) Max Threshold 64 (packets)
        ...

    Class-map: marked-network-control (match-any)
    ...
      Queueing
        Output Queue: Conversation 75
        Bandwidth 5 (%)
        Bandwidth 84 (kbps) Max Threshold 64 (packets)
       ...

    Class-map: marked-critical-data (match-any)
      ...
      Queueing
        Output Queue: Conversation 76
        Bandwidth 27 (%)
        Bandwidth 457 (kbps) Max Threshold 64 (packets)
       ...

    Class-map: marked-video (match-any)
     ...
      Queueing
        Output Queue: Conversation 77
        Bandwidth 15 (%)
        Bandwidth 254 (kbps) Max Threshold 64 (packets)
       ...

    Class-map: class-default (match-any)
      ...
      Queueing
        Output Queue: Conversation 78
        Bandwidth 23 (%)
        Bandwidth 390 (kbps) Max Threshold 64 (packets)
        ...

Thanks
netdoc01
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by:donmanrobb
donmanrobb earned 480 total points
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Can you post your full config or at least enough that shows your class-maps as well? It'll be easier for me to see what goes where if I can see everything your matching.
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by:donmanrobb
donmanrobb earned 480 total points
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However just based on the names of the policies, the bandwidth will not be used in times of congestion.

Just add max-reserved-bandwidth 100 to the interfaces where QoS is applied.
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Author Comment

by:netdoc01
ID: 21904746
Thanks
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