Eigrp metrics

Multiple metrics

EIGRP associates five different metrics with each route:

K1 = Bandwidth modifier 12
Minimum Bandwidth (in kilobits per second)

K2 = Load modifier
Load (number in range 1 to 255; 255 being saturated)

K3 = Delay modifier
Total Delay (in 10s of microseconds)

K4 = Reliability modifier
Reliability (number in range 1 to 255; 255 being the most reliable)

K5 = MTU modifier
Minimum path Maximum Transmission Unit (MTU) (though not actually used in the calculation)

By default, only total delay and minimum bandwidth are enabled when EIGRP is started on a router, but an administrator can enable or disable all the metrics as needed.

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

where the various constants (K1 through K5) can be set by the user to produce varying behaviors. An important and totally non-obvious fact is that if K5 is set to zero, the term  is not used (i.e. taken as 1).

The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably 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 Bandwidth and Delay metrics with following calculations:
Bandwidth for EIGRP = 107 / Interface Bandwidth
Delay for EIGRP = Interface Delay / 10

On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second. Dividing a value of 107 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth yields a value that is used in the weighted formula. Analogously, the interface delay is a configurable static parameter expressed in microseconds. Dividing this interface delay value by 10 yields a delay in units of tens of microseconds that is used 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.

EIGRP also maintains a hop count for every route, however, the hop count is not used in metric calculation. It is only verified against a predefined maximum on an EIGRP router (by default it is set to 100 and can be changed to any value between 1 and 255). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.