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EIGRP metric equation

Jun 16, 2011 1:04 PM

skcis 77 posts since
Nov 25, 2008

Hi all,

 

I have seen the following EIGRP equation numerous times in books (Cisco Press - BSCI)/documentation and on Cisco's site:

 

metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K3 * delay] * [ K5/(reliability + K4)]

 

We know that the default K values are 0, except for K1 and K3 (BW and delay respectively). The problem I'm having is that if K5 = 0, then doesn't that make the whole equation 0? I might be missing something  here, so please correct me if I'm wrong. And if the above was discussed some where, then please point me to that discussion.

 

Thanks!

  • LeeBrownUSA 102 posts since
    Apr 1, 2009
    Currently Being Moderated
    1. Jun 16, 2011 1:14 PM (in response to skcis)
    Re: EIGRP metric equation

    Look at K5 divided by reliability, zero divided by interger, should give you a non-zero value.

     

    metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K 3 * delay] * [ K5/(reliability + K4)]

  • Currently Being Moderated
    2. Jun 16, 2011 1:21 PM (in response to skcis)
    Re: EIGRP metric equation

    That is the equation, but if K5 is zero, then the term with K5 is not used.

     

    HTH

    DelVonte

  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    3. Jun 16, 2011 1:24 PM (in response to skcis)
    Re: EIGRP metric equation

    Aloha skcis,

     

    You are not going crazy.  If k5 is truely zero then the whole equation reduces to zero.  However, the EIGRP metric formula is "conditional", in that if K5 = 0 then the last term is "1" and thereby reduces the equation to:

     

    metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K 3 * delay]

     

    I have a nice MSexcel calculator under my profile as well as a JAVA calculator on my web site,

     

    http://www.btscomllc.com/calculator6_page.htm

     

    Check them out and give them a try.

     

    HTH

  • LeeBrownUSA 102 posts since
    Apr 1, 2009
    Currently Being Moderated
    4. Jun 16, 2011 1:26 PM (in response to DelVonte)
    Re: EIGRP metric equation

    I may be missing something but since when in a math equation can you throw out part of it?  Do we have any math wiz's here....;-)

  • Currently Being Moderated
    5. Jun 16, 2011 1:26 PM (in response to LeeBrownUSA)
    Re: EIGRP metric equation

    That is not true. Zero divided by anything is zero. So 0/x with x being any negative or positive number is zero. x/0 is undefined.

  • Currently Being Moderated
    6. Jun 16, 2011 1:28 PM (in response to LeeBrownUSA)
    Re: EIGRP metric equation

    Brian gives a great explanation here. This is not simply an equation, it is logic based. 

  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    7. Jun 16, 2011 1:37 PM (in response to LeeBrownUSA)
    Re: EIGRP metric equation

    Not a Math wiz, but see my post #3.

     

    The EIGRP formula is a "conditional" formula.  And you are correct, 0/X = 0  and then 0 x anything = 0.  So, the EIGRP formula must be "conditional".

     

    default k1= k3 = 1 and k2=k4=k5 =0

     

    interface parameters

    ===============

    bandwidth = 10Mbps

    delay = 1000 microseconds

    load = 1

    reliability = 255

     

    calculate the metric

     

    metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K 3 * delay] * [ K5/(reliability + K4)]

     

    metric = 281600

     

    Now set all k values to 1.

     

    k1=k2=k3=k4=k5=1

     

    interface parameters

    ===============

    bandwidth = 10Mbps

    delay = 1000 microseconds

    load = 1

    reliability = 255

     

    calculate the metric

     

    metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K 3 * delay] * [ K5/(reliability + K4)]

     

    metric = 1103

     

    HTH

  • LeeBrownUSA 102 posts since
    Apr 1, 2009
    Currently Being Moderated
    8. Jun 16, 2011 1:54 PM (in response to Brian)
    Re: EIGRP metric equation

    Ok, not a math wiz here either and other than an exam question are any of us using K values other than bandwidth and delay (K1, K3)? 

     

    Thanks Brian for a great explanation!

  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    10. Jun 16, 2011 2:41 PM (in response to skcis)
    Re: EIGRP metric equation

    No, indeed that last term is "x" or multiplication not addition.

     

     

    From a Cisco document,

     

     

    EIGRP uses these scaled values to determine the total metric to the network:

     

     

    metric = [K1 * bandwidth + (K2 * bandwidth) / (256 − load) + K3 * delay] * [K5 / (reliability + K4)]

     

    These K values should be used after careful planning. Mismatched K values prevent a neighbor

    relationship from being built, which can cause your network to fail to converge.

     

     

    If K5 = 0, the formula reduces to Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 − load) + k3 * delay].

     

     

    Mathematically, this means the last term must be a "1" for the formula to reduce in this manner.  Because of the "If k5 = 0" statement the formula is "conditional".

     

     

    HTH

  • Alex Presse 68 posts since
    Nov 25, 2008
    Currently Being Moderated
    11. Jun 16, 2011 2:36 PM (in response to skcis)
    Re: EIGRP metric equation

    The OCG guide is wrong. I have the same book and I know exactly what you're talking about. It's actually a known bug in the text - the addendums at ciscopress.com have the list of corrections to our books. I highly recommend periodically downloading the updates and revising your books!

  • Currently Being Moderated
    12. Jun 16, 2011 2:43 PM (in response to Alex Presse)
    Re: EIGRP metric equation

    Definitely agree with this. The older printings of the OCG contain several pages of typos, all of the corrections are on ciscopress.

  • RJ 32 posts since
    Jul 13, 2010
    Currently Being Moderated
    13. Jun 16, 2011 4:33 PM (in response to DelVonte)
    Re: EIGRP metric equation

    Hi Skcis,

     

    As Brian pointed out, the formula is a conditional one (logical). As far as the maths and formula goes, there are certain things which we can assume safely. Just as an example when you try to add a series ((1/n)+(1/n-1)+1), if the value of n is very large then the result approximates to 1. The value would be negligible and can be ignore is a safe assumption.

     

    Similarly the safe assumption or rule of thumb in EIGRP metric calculation as advocated by Cisco is when K5=0 the formula used is a modified version of the original formula (The last part involving K5 will become 1) . How they come to this assumption is not available as EIGRP is not an IETF standard so no specification is available. So you will have to take it as a rule of thumb and Cisco might have some good reasons to have made that assumption.

     

    EIGRP uses these scaled values to determine the total metric to the network:

    • metric = [K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]

    Note: These K values should be used after careful planning. Mismatched K values prevent a neighbor relationship from being built, which can cause your network to fail to converge.

    Note: If K5 = 0, the formula reduces to Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 - load) + k3 * delay].

     

     

    Please follow the link below to access the document which shows the section pased above.

     

    http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#eigrpmetrics

     

    Also read this article from ipexpert which gives more information about the K values of what it is and what it is not.

     

    http://blog.ipexpert.com/2010/03/03/eigrp-metric-k-values/

     

    Cheers!!

    RJ

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