Why we don't use or Prefer Load, Reliability & MTU to calculate EIGRP Metric?

[Ravi_916]

Why we don't use or Prefer Load, Reliability & MTU to calculate EIGRP Metric?

what is the actual Reason? 

[Deepak Kumar]

Hi,

Please note that MTU is not used in the calculation of the EIGRP metric. MTU is advertised with routing updates and stored in the topology database, and if your router has routes equal to the same destination and you have to choose one of them, MTU comes in to play.

Load: It changes based on traffic on the interface and is changing in seconds. As currently, the interface load is txload 1/255, rxload 1/255 and after a few seconds it can increase to txload 20/255, rxload 10/255. Therefore, the router has to recalculate the metric and it is a very CPU intensive process. It can cause an unstable network.

Reliability: A reliability metric is updated dynamically by the IOS. While calculating a composite metric to the destination, the EIGRP takes minimal reliability into consideration. It can be done by just comparing the reliability as advertised by the neighboring router to the reliability of an interface toward an advertising neighbor and also takes the lower value of both. In that, there is an essential fact regarding an EIGRP reliability metric handling. When the EIGRP advertising the information about path reliability as well as optionally factors that into a composite metric, the EIGRP does not send any updates when any changes in the interface reliability. In short, the change in the value interface's reliability will not trigger the sending EIGRP updates. A reliability metric of the route is simply the snapshot of its current reliability when it is last advertised.

[ngkin2010]

Add one more point, I think the change of interface's load will not cause triggered update.

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In short, the change in the value interface's reliability will not trigger the sending EIGRP updates. A reliability metric of the route is simply the snapshot of its current reliability when it is last advertised.

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So, your last sentence has point out the uselessness of using Load & Reliability in EIGRP.

[Richard Burts]

I would like to offer 2 comments about this. 

- the values for reliability and load are dynamic but EIGRP does not fluctuate as the values change. When EIGRP performs a route calculation it uses current values of these variables and that remains the values of the route until EIGRP needs to perform a new calculation. The route does not change just because reliability or load has changed. 

- these variables tell only about local conditions and do not provide anything useful about the path to the destination.