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!
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
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....;-)
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.
Brian gives a great explanation here. This is not simply an equation, it is logic based.
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
Thank you guys for all your input, especially Brian, LeeBrownUSA, and DelVonte.
I don't meant to loop around this discussion, but in OCG ROUTE (and I don't have this at hand now), however I do remember that in the book the formula is as follows:
metric = [ K1 * bandwidth + (K2 * bandwidth)/256 - load) + K 3 * delay] !!!+!!! [ K5/(reliability + K4)]
So the last term in the bracket with K5 is an addition/+ (I have placed exclaimation around it) . Now if we had K5=0, then the above would make senese for default values. So which formula is correct, the one with last bracket been added/+ or multiplied/*?
Thanks in advance!!
Andy
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
Definitely agree with this. The older printings of the OCG contain several pages of typos, all of the corrections are on ciscopress.
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:
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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