Router A is connected to Router B through a point-to-point T1 link. Router B is connected to network 192.168.100.0 on its Fast Ethernet interface. EIGRP is running on both routers. You install a second point-to-point link between the two routers for redundancy. The new link has a bandwidth of 256 kbps. Because the new link has a higher cost than the T1 link, the new link is not installed in the routing table and is idle. EIGRP only uses the T1 link to route to network 192.168.100.0. You want to load-balance between the two links. How do you enable EIGRP to load-balance between the two links?
By default, EIGRP can only load-balance equal-cost links and not load-balance between unequal-cost links. EIGRP needs to be configured to load-balance between unequal-cost links. The goal is to configure EIGRP to spread the traffic load inversely proportionally to the metrics on the two links. EIGRP uses the variance command to perform unequal-cost load balancing. The variance command defines a multiplier by which a metric can vary from the lowest-cost route. A variance of 1 means that the metrics of multiple routes must be equal.In this question, the metric of the T1 link is 1,657,856. The composite metric to network 192.168.100.0 (the total of the cost of the T1 link and the Fast Ethernet interface) is 2,172,416. The composite metric of the 256-kbps link is 10,514,432. To find the variance between the two paths to perform unequal-cost load balancing, divide the metric of the 256-kbps link by the T1 link: 10,514,432/2,172,416 = 4.8. Thus to configure unequal-cost load balancing, the variance on Router A needs to be set to 5, as follows:RouterA(config)#router eigrp 100RouterA(config-router)#variance 5