EIGRP Load balancing (Charter 15)

EIGRP Load balancing

Charter 15

One of the advantages of EIGRP is that, unlike OSPF and many other routing protocols, EIGRP also supports unequal-cost load balancing. You can set up your router to load balance over links with different metric to reach a destination. To accomplish unequal-cost load balancing, the variance command is used.

Imagine two offices in different locations connected via two links. One of the links has a better connection speed than the other. This is common in organisations as a form of redundancy or high availability. In this scenario, let’s assume the organisation also wants to actively use its backup link instead of only in the event of the primary link’s failure.

Example #1

ll three routers are running EIGRP. Routers R2 and R3 areconnected to the subnet 10.0.1.0/24. Both routers advertise the route to reach that subnet to R1. Router R1 receives the two routing updates for the subnet 10.0.1.0/24 with the same metric (the metric is the same because both routers connect to the subnet 10.0.1.0/24 and R1 across the links with the same bandwidth and delay values). Router R1 places both routes in the routing table and load balances across three links.

You can verify that R1 is indeed using both paths by typing the show ip route command

NOTE - the multiplier value, by default, is 1. The maximum value is 128.

Please look at the first graphic..

All three routers are running EIGRP. Routers R2 and R3 are connected to the subnet 10.0.1.0/24. Both routers advertise the route to reach that subnet to R1.
Router R1 chooses the route from R2 as the best route. Let's say that R1 calculated the metric of 40 for the path through R2. That route is placed in the R1's routing table. But what if we want to load balance traffic across the other link? The route through R3 has a feasible distance of 30, which is less than the metric of the successor route, so the feasibility condition has been met and that route has been placed in the R1's topology table.
Let's say that R1 calculated the metric of 60 for the route through R3. To enable load balancing across that link, you need to use the variance command:

(router-eigrp) variance MULTIPLIER

In this example, the variance 2 command can be used. This tells the router to load balance across any links with the metric less then 80 (because 40 times 2 is 80). The route through R3 is added to the routing table.

NOTE - a path has to be a feasible successor route to be used in unequal load balancing.

Let's do this in our GNS3

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Example #2

1.-Let's get our graphic working..

2.-Take a note in Notepad:It will make your life WAY easier...

3.-Let's fill up R1 and R3 first!

4-After that, we will do the same on R1

5.-Look how everything went up on the other interfaces

6.-Now take this notes into the notepad

7-we did R1, R2 and when we did R3 look at the reaction

Remember EIGRP uses the Diffusing Update ALgorithm (DUAL) which ensures a loop free routing domain by maintaining two separate routes in the eigrp topology table called Successor and Feasible Successor routes. The Successor route is the route that is injected into the routers routing table as the “best route” whereas the Feasible Successor route is effectively the backup route which is required to adhere to the successor feasibility condition.
The rule states that in order to be considered a Feasible Successor route, the advertised distance (AD) of the Feasible Successor should be less than the feasible distance (FD) of the Successor”
EIGRP maintains three separate tables, the neighbor table, topology table and the routing table;
The neighbor table establishes a list of all adjacent routers which a particular router has formed a neighbor relationship with. Neighbors exchange routing information and hello’s to ensure a neighbor is still up.

now show eigrp topology

So now the command to do Load balance is :

Now if do #show ip route and #show ip topology it will tell you that there is only one successor choosen, even thought 2 routes show up at the routing table