EIGRP Metrics

Composite Metric
Stable Metrics

As the following metrics are stable these are the metrics that are actually used for the calculation. If unstable metrics were to be used for the calculation then the network would be constantly changing and would not converge, using more CPU cycles to try and converge.

Stable Metrics are those that do not usually change from moment to moment. The bandwidth and delay of a connection/link will stay constant for the most part.

Bandwidth

EIGRP looks at the topology and the links. For the route that is to be used, whatever the slowest bandwidth is along that path is what is used as the bandwidth value for the metric calculation.

Delay

Delay is what the delay is across the whole path, it is all of the link delays added together. It is a cumulative metric.

Unstable Metrics

Metrics that are not constant. Reliability is calculated as connection/link uptime and will always be increasing or decreasing. Load is variable inline with how much traffic is on the network.

Reliability

How long the connection has been up for, if a line is flapping then it will not be considered reliable.

Load

Load is how busy the line is. If you have a 1Gbps connection that is 99% utilised and a 5Mbps connection that is 3% utilised. Then load would prefer the slower but also less utilized line as there is more available bandwidth there.

Maximum Transmission Unit (MTU)

MTU was useful when connection speeds were low (dial-up connections etc.) and the MTU was actually a consideration for how packets were to be routed. Nowadays connection speeds are high enough that it is not a factor that needs to be taken into account. Although it is sent in packets (UPDATES etc.) MTU is not used in the EIGRP formula, you cannot turn it on even if you wanted to.

Diffusing Update ALgorithm (DUAL).

Actual name is ‘Diffusing Update Algortihm – Finite State Machine’

You can adjust the metric weights (K Values) using the following:

router(config)# router eigrp 1
router(config-router)# metric weights <ToSalways = 0> <K1> <K2> <K3> <K4> <K5>

*** ToS must be set to 0 otherwise EIGRP will not work ***

The default K values are:

When the default K values are in place the formula is different. This is because there are three values that default to zero (K2, K4  & K5) which removes them from the equation

The Cisco recommendation is to not alter the metric weight values. If you make changes to the metric weights then you will need to change the weights on all routers in the autonomous system (AS) otherwise the route calculations will be different on differing routers. If K values do not match from one router to another then they will not form adjacencies and existing adjacencies will be dropped.

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