EIGRP Metric confusion

rakem

I'm trying to get my head around the metric calculation for EIGRP.

So the way i see it is as follows:

Default equation is:

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

ok now if we are using the default K values which are k1, k3 = 1 and k2, k4, k5 = 0 then the equation would look like this

M = [1 * bandwidth + (0 * bandwidth) / (256 - load) + 1 * delay] * [0 / (reliability + 0)]

which can be simplified down to this:

M = bandwidth / (256 - load) + delay

Is that correct so far?

I have been reading from this site
http://www.cisco.com/warp/public/103/eigrp-toc.html#theoryofoperation

and it says that the equation can be simplified down to:

metric = bandwidth + delay

Now with my example above, the simplified equation was:
M = bandwidth / (256 - load) + delay

so im assuming that on the cisco site the equation is metric = bandwidth + delay because by default EIGRP only uses bandwidth and delay to compute the metric, so this is why the (256 - load) is not in the equation.... is this correct?

thanks

EdTheLad

rakem wrote:

Default equation is:

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

K2*BW/(256- load)

K2=0
0*BW=0
0/(256-load) = 0

rakem

ah i see....

equation should be written like this

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

ok so now with the example from the link above, and using the default m = bandwidth + delay equation:

b=(10000000/lowest bandwidth) x 256
d=(sum of all delays) x 256

the first example they give has min bandwidth = 56 and sum of delays = 2200

so that would give us

b=(10000000/56) x 256
b=178571 x 256
b=45714176

d=2200 x 256
d=563200

M = b+d
M = 45714176 + 563200
M = 46277376

That looks right to me.... i just hate they way they write it all on one line on the cisco site, its confusing.
and once this has been done for all possible routes, the route with the lowest metric is placed in the routing table and the metric is now the feasible distance, correct?


So in the BSCI exam will there be questions where i have to actually calculate the metric?

Paul Boz

I highly doubt you'll have to calculate the metric down to the formula. The exams are more about knowing how something like bandwidth or delay values can be manipulated to influence routing than being able to recall the specific formula. The logic is that to understand the more complex question you have to know the fundamentals (the formula, for example).