Unequal-cost

Unequal-cost

EIGRP can load balance traffic via paths with unequal cost. This is achieved by configuring the variance.

It multiplies against the best metric.

If the Successor has an FD of 10 and I set a variance of 2, then any path between 10-20 is now considered equal. Note that variance only work for paths that are FS. If redundant path exists, but it's not an FS, the variance will not make it an FS then put it in the routing table.

To check how variance works, let's consider the route from R1 to 10.2.4.0/24, which the connection between R2 and R4.

R1#sho ip eigrp topology | be 10.2.4

P 10.2.4.0/24, 1 successors, FD is 3072

        via 10.1.2.2 (3072/2816), GigabitEthernet1

As we can seem no feasible successor exist, hence variance won't make any change here. First, let's find ways to make the path via R3 as an FS. We will do it by configuring offset-list on R2 to make increase the FD at R1 and make the RD from R3 to R1 match the feasibility condition. 

First, we will analyze all links in R1's topology table to see the alternate path values. As we can see below, the path via R3 does not meet the feasibility condition.

R1#sho ip eigrp topology all | be 10.2.4

P 10.2.4.0/24, 1 successors, FD is 3072, serno 156

        via 10.1.2.2 (3072/2816), GigabitEthernet1

        via 10.1.3.3 (3328/3072), GigabitEthernet2

Now we will login to R2 and configure an offset-list to increase the FD for 10.2.4.0 at R1.

R2#conf t

R2(config)#access-list 1 permit 10.2.4.0 0.0.0.255

R2(config)#router eigrp 1

R2(config-router)#offset-list 1 out 100 gigabitEthernet 1

Now lets see the topology table on R1. As we can see below, now the path via R3 is an FS.

1#sho ip eigrp topology | be 10.2.4

P 10.2.4.0/24, 2 successors, FD is 3172

        via 10.1.2.2 (3172/2916), GigabitEthernet1

        via 10.1.3.3 (3328/3072), GigabitEthernet2

Before changing the variance, we will see the routing table of R1 to check the routers it contains. As you can see below, only one path exists towards 10.2.4.0/24.

R1#sho ip route | be 10.2.4.0

D        10.2.4.0/24 [90/3172] via 10.1.2.2, 00:00:09, GigabitEthernet1

D        10.3.4.0/24 [90/3072] via 10.1.3.3, 00:00:09, GigabitEthernet2

Now lest change the variance and see the result. One of the simple ways to find the value for the variance is to do the best path FD/FS RD, in this case 3172/3072. If the value is not exact value, then round up by excess. Example, if the result is 1.234, it will be 2. So 3172/3072 = 1.032 ~ 2, so the variance is 2.

Now we will change the variance on R1. 

R1#conf t

R1(config)#router eigrp 1

R1(config-router)#variance 2

After changing the variance, we can see from the below output that there are now two unequal cost paths towards 10.2.4.0/24.

R1#sho ip route | be 10.2.4

D        10.2.4.0/24 [90/3328] via 10.1.3.3, 00:00:15, GigabitEthernet2

                     [90/3172] via 10.1.2.2, 00:00:15, GigabitEthernet1

Below is a more detailed output of show ip route with the destination route specified. As we can see two paths exist and the share count is also observed.

R1#sho ip route 10.2.4.0

Routing entry for 10.2.4.0/24

  Known via "eigrp 1", distance 90, metric 3172, type internal

  Redistributing via eigrp 1

  Last update from 10.1.3.3 on GigabitEthernet2, 00:00:21 ago

  Routing Descriptor Blocks:

    10.1.3.3, from 10.1.3.3, 00:00:21 ago, via GigabitEthernet2

      Route metric is 3328, traffic share count is 229

      Total delay is 30 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

  * 10.1.2.2, from 10.1.2.2, 00:00:21 ago, via GigabitEthernet1

      Route metric is 3172, traffic share count is 240

      Total delay is 23 microseconds, minimum bandwidth is 1000000 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hop