Answers: EIGRP Unequal Cost Routes

This latest lab lets you exercise your basic EIGRP config skills, with a few twists. Does the actual link speed matter? Do you recall how to use wildcard masks in your EIGRP config? And how do you use multiple unequal-metric routes? Check out the original requirements, try your answer, and then check back here.

Answers

Figure 1: Router Diamond with IP Subnets

Example 5: R1 Config

router eigrp 40
 network 10.0.0.0
 network 20.0.0.0
 network 192.168.1.0

Example 6: R2 Config

router eigrp 40
 network 10.0.0.0
 network 30.0.0.0
 network 40.0.0.0

Example 7: R3 Config

router eigrp 40
 network 20.0.0.0
 network 30.0.0.0
 network 50.0.0.0

Example 8: R4 Config

router eigrp 40
 network 40.40.40.0 0.0.0.255
 network 50.50.50.0 0.0.0.255
 network 192.168.2.0 0.0.0.255
 variance 3

Commentary

First, none of the routers had EIGRP configured yet, so the configuration on all the routers enables EIGRP. In this case, routers R1, R2, and R3 have similar configuration. All enable EIGRP using the same ASN (because the same ASN is required) using command router eigrp 40. Each uses three network subcommands, with each command referencing a classful network. As a result, R1, R2, and R3 have enabled EIGRP on all interfaces shown in the figure, plus any interfaces added in the future which happen to be assigned addresses in those same classful networks.

As requested, R4 instead matches only addresses in the same subnet with its network commands, like the network 40.40.40.0 0.0.0.255 command. This command matches all interfaces whose address begins 40.40.40, which includes only that one interface (G0/3). Future interfaces added to R4 that uses other addresses in network 40.0.0.0 will not be enabled by default.

Next, note the variance 3 command on router R4. This command tells R4 that:

Include any feasible successor routes in the routing table if their composite metric is <= 3 times the metric of the best route.

That meets the requirement stated in this lab.

However, note that a variance of 3 does not provide enough room for R4 to have two routes. Ethernet links on routers adjust their default bandwidth settings based on the current link speed. In this case, the R3-R4 link was pre-set to run at 100 Mbps, causing both to default to a bandwidth of 100 Mbps. As a result, R4’s 2^nd best route is roughly 10X worse than its best route.