CCIE Level question

tunerXtunerX Member Posts: 447 ■■■□□□□□□□
in CCIE
Calculating flows for WFQ.

Flow 1 - 500 byte packets with IP precedence 3.
Flow 2 - 250 byte packets with IP precedence 3.
Flow 3 - 250 byte packets with IP precedence 1.

For every 1 packet of flow 1 that is serviced, how many packets of flow 2 and flow 3 will be serviced?
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  • Ricka182Ricka182 Member Posts: 3,359
    I have no clue, but my guess is:

    Uhhh, WFQ=WTF!!! icon_confused.gificon_confused.gificon_confused.gif

    1/2 packet for flow 2,and 1/2 for flow 2...????? Sorry, I have never touched a Cisco book, I just like trivia...
    i remain, he who remains to be....
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  • viper75viper75 Member Posts: 726 ■■■■□□□□□□
    mama!!! icon_sad.gif
    CCNP Security - DONE!
    CCNP R&S - In Progress...
    CCIE Security - Future...
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  • DrakonblaydeDrakonblayde Member Posts: 542
    tunerX wrote:
    Calculating flows for WFQ.

    Flow 1 - 500 byte packets with IP precedence 3.
    Flow 2 - 250 byte packets with IP precedence 3.
    Flow 3 - 250 byte packets with IP precedence 1.

    For every 1 packet of flow 1 that is serviced, how many packets of flow 2 and flow 3 will be serviced?

    I'm going to assume the weight being used is post IOS 12.0(5)T.

    Let's see, the weight for flow 1 would be....

    32384 / 2000 = 16.192

    Flow 2 would be

    32384 / 1000 = 32.384

    Flow 3 would be

    32384 / 500 = 64.768


    So for every one packet that flow 1 gets through, flow 2 will get two through and flow 3 will get 4 through.

    At least, I think :) QoS isn't something I'm delving into in depth at present.
    = Marcus Drakonblayde
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  • Orion82698Orion82698 Member Posts: 483
    I'm going to assume the weight being used is post IOS 12.0(5)T.

    Let's see, the weight for flow 1 would be....

    32384 / 2000 = 16.192

    Flow 2 would be

    32384 / 1000 = 32.384

    Flow 3 would be

    32384 / 500 = 64.768


    So for every one packet that flow 1 gets through, flow 2 will get two through and flow 3 will get 4 through.

    At least, I think icon_smile.gif QoS isn't something I'm delving into in depth at present.

    Wow! Man...... I thought addition was hard icon_wink.gif
    WIP Vacation ;-)

    Porsche..... there is no substitute!
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  • DrakonblaydeDrakonblayde Member Posts: 542
    hehe, I'm not sure if I'm right, I remember reading over Weighted Fair Queueing in one of my books, and I had to look up the weight for the post 12.0(5)T IOS (pre it's 4096), but after that, it's a fairly simple formula...


    Weight = 32384 ( or 4096) / (IP Precedence + 1) * PacketSize

    After that, just do the math and whatever comes out with the higher weight has more priority
    = Marcus Drakonblayde
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  • tunerXtunerX Member Posts: 447 ■■■□□□□□□□
    You got the formula right but the weights backwards. With WFQ lower weights are better.

    flow 1: 32384/(3+1)=8096
    flow 2: 32384/(3+1)=8096
    flow 3: 32384/(1+1)=16192

    If we assume that all flows started at the same time. The initial value of the SN (sequence numbers) for each flow will be zero. To make things easier, I will drop all of the numbers on the right.

    flow 1: SN1=(8096*500)=4048000
    flow 2: SN1=(8096*250)=2024000
    flow 3: SN1=(16192*250)=4048000

    all following serial numbers are calculated by adding the previous serial number to the product of weight and length.

    SN = (previous SN + (weight*length))


    On average for every 1 packet that flow 1 sends, flow 2 will have sent 2 and flow 3 will send 1.

    Queue
    F1: SN3~12, SN2~8, SN1~4
    F2: SN3~6, SN2~4, SN1~2
    F3: SN3~12, SN2~8, SN1~4
    Queue
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  • DrakonblaydeDrakonblayde Member Posts: 542
    tunerX wrote:
    You got the formula right but the weights backwards. With WFQ lower weights are better.

    DOH! Well, I'll never forget that little tidbit again ;)

    Hrm, my math was also wrong. I guess the proper way for the formula then would be Weight = (32384 / (IP Precedence + 1)) * PacketSize.
    = Marcus Drakonblayde
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