EIGRP Unequal Cost Load Balancing

Just like OSPF or RIP, EIGRP can do load balancing but it has one more trick in its hat. RIP and OSPF both can do load-balancing but the paths have to be equal.

EIGRP can do something cool…unequal load-balancing! Even better it will share traffic in a proportional way, if you have a feasible successor that has a feasible distance which is 5 times worse than the successor then traffic will be shared in a 5:1 way.

Let’s take a look at an example of how EIGRP can do load balancing:

We’ll view this topology from R1’s perspective. Let’s fill in the successor, feasible successor, advertised and feasible distance in a table. If you have no idea how this works please read my introduction to EIGRP first.

	Advertised Distance	Feasible distance
R2	15	20
R3	10	15	SUCCESSOR
R4	14	114	FEASIBLE SUCCESSOR

This is our first example where we found out the successor and feasible successor. If you look at the routing table you will only find the successor there. Now we are going to change things so we’ll see the feasible successor in the routing table as well so it will load-balance.

You can do this by using the variance command. The variance command works as a multiplier:

• Our successor has a feasible distance of 15.
• Our feasible successor has a feasible distance of 114.

In order to load-balance our feasible successor needs to have a feasible distance which is less or equal than the successor X multiplier.

If we set the variance at 2, this is what we get:

Feasible distance of successor is 15 x 2 (multiplier) = 30.

114 is higher than 30 so we don’t do any load-balancing.

If we set the variance at 5, this is what we get:

Feasible distance of successor is 15 x 5 (multiplier) = 75.

114 is still higher than 75 so still no load-balancing here.

Now I’m going to set the variance at 8 and this is what we get:

Feasible distance of successor is 15 x 8 = 120.

114 is lower than 120 so now we will put the feasible successor in the routing table and start load-balancing!

Are we ever going to use the route through R2? No, we won’t since it’s not a feasible successor!

I hope this explanation is useful to you. Want to see this in action? Take a look at the variance configuration example.

If you have any questions please leave a comment.