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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:
[teaser]

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 lower feasible distance 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.

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Tags: Load Balancing, Successor, Variance

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hussien.samer says:

Hi Rene,

If we have three path to destination and we configure variance to use the three path just like in your example here :-

NetworkLessons.com – 24 Jun 13

EIGRP Variance Command Example | NetworkLessons.com

This tutorial demonstrates the EIGRP Variance command to enable load balancing over feasible successors.

My question is :-
The traffic will be shared in which way ? to be more clarify, In the begging of this article you said " if the feasible successor has a feasible distance which is 5 times worse than the successor then traffic will be shared in a 5:1 way " so what the rate will be if we have three path to destination ( the successor and tow feasible successors ) ?

i didn’t understand this
could you explain more details about this . that will be hard when the numbers are high .
and i show command its show me share count

Router#show ip route 2.2.2.2
Routing entry for 2.2.2.0/24
  Known via "eigrp 1", distance 90, metric 156416, type internal
  Redistributing via eigrp 1
  Last update from 192.168.0.1 on FastEthernet0/0, 00:01:11 ago
  Routing Descriptor Blocks:
    192.168.0.1, from 192.168.0.1, 00:01:11 ago, via FastEthernet0/0
      Route metric is 156416, traffic share count is 240
      Total delay is 5110 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
  * 11.11.11.2, from 11.11.11.2, 00:01:11 ago, via Serial5/0
      Route metric is 640256, traffic share count is 59
      Total delay is 5010 microseconds, minimum bandwidth is 5000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

Rene Molenaar September 17, 2015 at 13:29 #
Hi Hussein,

It will be shared in proportion based on the feasible distance. If the successor had a FD of 100 and the feasible successors had a FD of 20 and 10 then you’d see a 10:2:1 ratio.

Rene

azmuddincisco says:

Thanks a lot…you the man…

Azm
lagapides says:

Hello Florain

florianl:

he FS normally is not inserted in the routing table, only the successor route can be found there.

The FS is a FS only if it is not in the routing table. If it is in the routing table it has the best metric and is thus considered a successor, even if there are more than one such as is the case with EIGRP load balancing.

florianl:

If you do unequal-cost LB, the FS routes will be put in the routing table, right?

If you do unequal cost LB, then the routes in the routing table are also considered successors and not feasable successors. But putting aside terminology, the answer is yes, the routes that were FS (at least the next one(s) in line) is inserted into the routing table and becomes a successor.

florianl:

And if there are multiple FS in the EIGRP topology table, what is the deciding factor to choose one to replace the successor route in case it fails? The one with the lowest AD becomes the next successor?

The deciding factor of choosing the next successor is always the same, regardless of whether or not you are using load balancing. EIGRP will keep up to six feasible successors in the topology table and only the one (or ones) with the best metric (or metrics) become the successor(s) and are placed in the routing table.

I hope this has been helpful!

Laz
lagapides says:

Hello Marcel

All routes learned via EIGRP are contained within the topology table, whether they are actively being used (in the routing table) or not. These routes are examined by EIGRP and only the route with the best metric is placed within the routing table. The rest of the routes still exist in the topology table however.

So, the routing table will only have a single route to a particular destination by default (unless two routes have an equal metric of course). In order to determine if a route can be used for unequal load balancing, the variance value is used and the calculation as described above is implemented on the metrics found in all routes within the topology table. If the metric of the route fulfills the metric * variance > FD condition, then it is then installed in the routing table as well, as a secondary, unequally load balanced route.

So the topology table is used in this whole evaluation of routes because it is there that all the available routes and their metrics are found.

I hope this has been helpful!

Laz

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