Unequal cost loadbalancing with EIGRP
I find it both good to know and very likely that tasks like this will be tested in the ccie lab. So I did some extra training to become more comfortable with this kind of tasks with EIGRP. The task in my example is just taken out of nowhere, but Im going to solve it with a general approach.
Make sure that traffic to the network depicted as a switch is loadbalanced from Router E between both paths, and Path A should be used 6 times as much as the other! The routers are configured with metric weights 0 0 1 0 0, in other words, only k3=1, so delay would be the only parameter for metric calculations.
1. Make sure both paths are used
In this case, the primary path would have a feasible distance of 30. That means the secondary path wont be imported to the topology table since the Reported Distance from Router D is too high. To pass the feasibility check the reported distance of the secondary path must be lower than the current Feasible Distance from Router E’s perspective. To do this, we have to start by adding some metric distance to the primary path in order to make the FD higher than D’s RD.
We have to add delay to the interface of path A to router E so the total delay is more than 50!
verification: check EIGRP topology table, after we added some delay, the prefix got imported to EIGRP topology table. Now we see that both paths are there when we passed the Feasibility Check.
2. Metric ratio
Since we wanted the router to loadbalance between these two with a given ratio, we need to configure the metrics to suit this ratio. So lets add delay to Path B so the new total delay of that path is 6 times more than A. As we see in the picture above, both paths now have a total delay of 60. Lets multiply this by 6 = 360. This would be the new total delay. The reported distance was 50.
60*6-50=310. Lets add 310 to the interface towards path b.
Now we have the other path in our topology table and with 6 times higher metric than path a. perfect.
Now lets add variance to our eigrp configuration:
RouterE(config)#router eigrp 100
Finally, doublecheck the routingtable:
Both paths are there, the traffic share count is 1 on our primary route and 6 on the secondary. Done.