CCNP ROUTE: 8. EIGRP Unequal Load Balancing

It's used for traffic balancing and instant convergence, since the "backup" route is already in the routing table.

For configuring load balancing, we must use two commands: maximum-paths and variance.
"maximum paths" - defines the maximum allowed number of routes in the routing table for each destination network (default 4)
"variance" - tell IOS that the EIGRP metrics can be close in value and still be considered worthy of being added to the routing table (default 1)

The router multiplies the variance (1-128) times the successor route's FD (best metric route to reach the destination).
Any feasible successor (FS) routes whose metric is less than the product of variance times the FD are considered to be equal routes and may be placed into the routing table.

Rule: IF (FS < variance*FD) THEN place in the routing table

Consider R4's routes to the 172.16.0.4 network:
R4#show ip eigrp topology 
...
P 172.16.0.4/30, 2 successors, FD is 2195456
        via 172.16.0.1 (2195456/281600), Serial0/0
        via 172.16.0.14 (2195456/281600), Serial0/2
R4#show ip route
...
D       172.16.0.4/30 [90/2195456] via 172.16.0.14, 00:19:37, Serial0/2
                      [90/2195456] via 172.16.0.1, 00:19:37, Serial0/0

Having the default variance of 1 and equal cost paths to 172.16.0.4/30, R4 added both routes in the routing table.

Consider R4's unequal cost routes to the 172.16.0.8 network:
R4#show ip eigrp topology 
...
P 172.16.0.8/30, 1 successors, FD is 2681856
        via 172.16.0.1 (2681856/2169856), Serial0/0
        via 172.16.0.14 (2707456/2195456), Serial0/2
R4#show ip route
...
D       172.16.0.8/30 [90/2681856] via 172.16.0.1, 00:19:36, Serial0/0

For having both routes in the routing table, we must configure the variance parameter on R4:

R4(config)#router eigrp 1
R4(config-router)#variance 2
R4#show ip route
...
D       172.16.0.8/30 [90/2707456] via 172.16.0.14, 00:00:07, Serial0/2
                      [90/2681856] via 172.16.0.1, 00:00:07, Serial0/0

The result of the EIGRP calculation is obvious: 2707456 < 2*2681856.