EIGRP - metric equation

When implementing EIGRP it is important to understand how the metric is calculated using the EIGRP equation. There are various forms used in different contexts, and the following should help to clear up the differences between them.

The classic EIGRP metrics formula is the following:

Metric=[K1×Bandwidth+(K2×Bandwidth)(256Load)+K3×Delay]×[K5/(Reliability+K4)]\text{Metric} = \left[K1 \times \text{Bandwidth} + \frac{(K2 \times \text{Bandwidth})}{(256 - \text{Load})} + K3 \times \text{Delay}\right] \times \left[K5 / (\text{Reliability} + K4)\right]

The above is based on the IGRP metrics formula. This is done to make EIGRP backward compatible with IGRP. For EIGRP, the result should be multiplied by 256 to change the metric from a number represented using 24-bits to a number represented using 32-bits. The resulting formula is:

Metric=256×[K1×Bandwidth+(K2×Bandwidth)(256Load)+K3×Delay]×[K5/(Reliability+K4)]\text{Metric} = 256 \times \left[K1 \times \text{Bandwidth} + \frac{(K2 \times \text{Bandwidth})}{(256 - \text{Load})} + K3 \times \text{Delay}\right] \times \left[K5 / (\text{Reliability} + K4)\right]

By default K1 = K3 = 1 and K2 = K4 = K5 = 0. This reduces the equation to the following:

Metric=256(107MinBandwidth+TotalDelay10)Metric = 256 * \left(\frac{10^7}{MinBandwidth} + \frac{TotalDelay}{10}\right)

EIGRP wide metrics are used to accommodate the ever increasing speeds of interfaces. The wide metrics formula is:

WideMetric=[(K1BW+K2BW256 Load +K3Latency+K6Extended)K5 K4+ Reliability ]Wide Metric =\left[\left(\mathrm{K}_1{ }^* \mathrm{BW}+\frac{\mathrm{K}_2{ }^* \mathrm{BW}}{256-\text { Load }}+\mathrm{K}_3{ }^*\right.\right. Latency +\mathrm{K}_6{ }^*Extended \left.) * \frac{\mathrm{K}_5}{\mathrm{~K}_4+\text { Reliability }}\right]

Just as EIGRP is scaled up by 256 to accommodate IGRP, EIGRP wide metrics is scaled up by 65535 to accommodate classic EIGRP. SO the formula becomes:

 Wide  Metric =65,535[(K1107 Min. Bandwidth +K2107 Min. Bandwidth 256 Load +K3 Latency 106+K6Extended)K5 K4+ Reliability ]\begin{aligned} & \text { Wide } \\ & \text { Metric }\end{aligned}=65,535 *\left[\left(\frac{\mathrm{K}_1{ }^* 10^7}{\text { Min. Bandwidth }}+\frac{\frac{\mathrm{K}_2{ }^* 10^7}{\text { Min. Bandwidth }}}{256-\text { Load }}+\frac{\mathrm{K}_3{ }^* \text { Latency }}{10^{* 6}}+\mathrm{K}_6{ }^*\right.\right. Extended \left.) * \frac{\mathrm{K}_5}{\mathrm{~K}_4+\text { Reliability }}\right]

When calculating the metric, the way in which EIGRP computes the metrics must be taken into account.