EIGRP Wide Metrics

Along with the EIGRP multi-af (named) mode also came the Wide Metrics of EIGRP. Let us do a short recap of the classic metrics before tackling the wide metrics.

The classic metrics consisted of K values 1 through 5 with the default settings listed below.

K1 = bandwidth = 1
K2 = load = 0
K3 = delay = 1
K4 = reliability = 0
K5 = MTU = 0

So, the above means that any K value set to 1 i used in the metric calculation - so by default only bandwidth and delay is used. These K values are used in a formula (shown at the bottom of this post) to scale the bandidth, load, delay etc. and finally produce a number that is the actual metric for EIGRP.

In EIGRP Named Mode, the wide metrics was introduced along with a sixth K value. The delay was also changed from being measured in tens of microseconds to being measured in picoseconds. The new wide metrics, delay in picoseconds and K6 are only part of EIGRP multi-af mode - it is not available in classic mode.

The need for wide metrics, in short, is due to the classic metrics being unable to handle interfaces above 1 Gigabit properly. Due to both the bandwidth and delay values not allowing for the granularity needed. So the wide metrics introduce a 64-bit metric calculation as opposed to the classic 32-bit.

The new 64-bit calculations introduced another issue - the metric in the RIB can only accommodate 4 bytes (32 bits) of data. This is solved by scaling the metric for the RIB using the metric rib-scale <1-255> command in the routing process address-family section.

Below is an excerpt of the Cisco documentation regarding the EIGRP metrics calculation formula.

Use this command to alter the default behavior of EIGRP routing and metric computation and to allow the tuning of the EIGRP metric calculation for a particular type of service (ToS). 

If k5 equals 0, the composite EIGRP metric is computed according to the following formula:
metric = [k1 * bandwidth + (k2 * bandwidth)/(256 – load) + k3 * delay + K6 * extended metrics] 

If k5 does not equal zero, an additional operation is performed:
metric = metric * [k5/(reliability + k4)] 

Scaled Bandwidth= 10^7/minimum interface bandwidth (in kilobits per second) * 256 

Delay is in tens of microseconds for classic mode and pico seconds for named mode. In classic mode, a delay of hexadecimal FFFFFFFF (decimal 4294967295) indicates that the network is unreachable. In named mode, a delay of hexadecimal FFFFFFFFFFFF (decimal 281474976710655) indicates that the network is unreachable.  

Reliability is given as a fraction of 255. That is, 255 is 100 percent reliability or a perfectly stable link. 

Load is given as a fraction of 255. A load of 255 indicates a completely saturated link.

A thing to remember is, that if you change the K value weighting on one router, you will have to change it on all routers in the EIGRP network. The K values must match!