Continue the discussion forum.networklessons.com
57 more replies!
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 loadbalancing but the paths have to be equal.
EIGRP can do something cool…unequal loadbalancing! 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 loadbalance.
You can do this by using the variance command. The variance command works as a multiplier:
[teaser]
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 loadbalancing.
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 loadbalancing 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 loadbalancing!
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.
57 more replies!
Hi Rene,
If we have three path to destination and we configure variance to use the three path just like in your example here :
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
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
REPLY
Thanks a lot…you the man…
Azm
Hello Florain
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.
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.
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
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