EIGRP classic metrics. Common confusion?

Hi,

Studying EIGRP wide metrics I came across a question:

  • For classic metrics, If bandwidth metric is 10^7/BW*256 why is it that for BW=10Gbps -> metric = 256 and for BW=20Gbps -> metric =256?? Why not 128? (this question applies to delay below 10uS although not considered)
  • And why is it that for wide metrics BW=10Gbps -> metric =65536 and BW=20Gbps -> metric = 32768? Why not 65536 too?
  • Why?

From what I have looked at, EIGRP classic metrics are the IGRP simply multiplying by 256... first calculate all "single" metrics, no floating numbers and only after that multiply by 256.

The confusion to me, seems to come from lots of cisco documentation, The EIGRP Info RFC (and consequently the INE ATC video).

From draft-savage-eigrp-02:

5.5 5.5 EIGRP Metric Calculations
5.5.1 Classic Metrics
One of the original goals of EIGRP was to offer and enhance routing
solutions for IGRP. To achieve this, EIGRP used the same composite
metric as IGRP, with the terms multiplied by 256 to change the metric
from 24 bits to 32 bits.


...
...
 5.5.1.1 Classic Composite Formulation
EIGRP calculates the composite metric with the following formula:

metric = {K1*BW+[(K2*BW)/(256-load)]+(K3*delay)}*{K5/(REL+K4)}

...
...
 Bandwidth is the inverse minimum bandwidth (in kbps) of the path in
bits per second scaled by a factor of 256 multiplied by 10^7. The
formula for bandwidth is

(256 x (10 ^ 7))/BWmin

The delay is the sum of the outgoing interface delay (in
microseconds) to the destination. A delay set to it maximum value
(hexadecimal FFFFFFFF) indicates that the network is unreachable. The
formula for delay is

[sum of delays] x 256

 

Seems to me that the very first paragraph is many times ignored leading to very strange maths and considering the factor of 256 as part of BW or delay in classic metrics is just wrong.

The correct form of expressing the EIGRP classic metrics formula is:

EIGRP composite cost metric = 256*((K1*Scaled Bw) + (K2*Scaled Bw)/(256 – Load) + (K3*Scaled Delay)*(K5/(Reliability + K4)))

Scaled Bw = (107/minimum bandwidth (Bw) in kilobits per second)

Scaled Delay = (Delay/10)

as seen on EIGRP Wide Metrics and never considering the 256 as part of the components.

 

Just posting my thoughts and hoping to read yours on this subject. What do you think?

 

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