Editing Enhanced Interior Gateway Routing Protocol (section)

====Routing metric====
The composite routing metric calculation uses five parameters, so-called K values, K1 through K5. These act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc.

By default, only total delay and minimum bandwidth are considered when EIGRP is started on a router, but an administrator can enable or disable all the K values as needed to consider the other Vector metrics.

For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:

: <math>\bigg [ \bigg ( K_1 \cdot {\text{Bandwidth}}_{E} + \frac{K_2 \cdot {\text{Bandwidth}}_{E}}{256-\text{Load}} + K_3 \cdot {\text{Delay}}_{E}
                      \bigg )
         \cdot \frac {K_5}{K_4 + \text{Reliability}} \bigg ] \cdot 256</math>

where the various constants (<math>K_1</math> through <math>K_5</math>) can be set by the user to produce varying behaviors. An important and unintuitive fact is that if <math>K_5</math> is set to zero, the term <math>\tfrac {K_5}{K_4 + \text{Reliability}}</math> '''is not used (i.e. taken as 1)'''.

The default is for <math>K_1</math> and <math>K_3</math> to be set to 1, and the rest to zero, effectively reducing the above formula to <math>({\text{Bandwidth}}_{E} + \text{Delay}_{E}) \cdot 256</math>.

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent [[routing loop]]s may result. Cisco routers running EIGRP will not form an EIGRP adjacency and will complain about K-values mismatch until these values are identical on these routers.

EIGRP scales the interface ''Bandwidth'' and ''Delay'' configuration values with following calculations:
: <math>{\text{Bandwidth}}_{E}</math> = 10<sup>7</sup> / Value of the ''bandwidth'' interface command
: <math>{\text{Delay}}_{E}</math> = Value of the ''delay'' interface command

On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second (setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 10<sup>7</sup> kbit/s (i.e. 10&nbsp;Gbit/s) by the interface bandwidth statement value yields a result that is used in the weighted formula. The interface delay is a configurable static parameter expressed in tens of microseconds. EIGRP takes this value directly without scaling into the weighted formula. However, various ''show'' commands display the interface delay in microseconds. Therefore, if given a delay value in microseconds, it must first be divided by 10 before using it in the weighted formula.

[[IGRP]] uses the same basic formula for computing the overall metric, the only difference is that in IGRP, the formula does not contain the scaling factor of 256. In fact, this scaling factor was introduced as a simple means to facilitate backward compatility between EIGRP and IGRP: In IGRP, the overall metric is a 24-bit value while EIGRP uses a 32-bit value to express this metric. By multiplying a 24-bit value with the factor of 256 (effectively bit-shifting it 8 bits to the left), the value is extended into 32 bits, and vice versa. This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically.