EIGRP Unequal Cost Routes

#EIGRP uses a complex metric calculation. Most of us do not memorize the math formula, instead remembering the metric components: slowest bandwidth in the route and cumulative delay for all links. This lab lets you ponder how to configure EIGRP to use multiple unequal-metric routes, which begs many questions about what metrics EIGRP would calculate, and about how to configure correctly to make sure you use those multiple routes.

Requirements

First, enable EIGRP on all routers and interfaces shown in the figure. The lab begins with the IPv4 addresses configured; your job is to add EIGRP configuration, with a few specific requirements.

Additionally, configure the network so that R4 will add routes for both the left route (through R2) and the right route (through R3) to the LAN off R1. To do so, rather than making you remember the math and predict the computed metric of each route, assume that the composite metric for the worse-metric route is less than 3X the metric for the best route.

The specific rules for this lab are:

• Enable EIGRP on all routers
• Use ASN 40
• Allow unequal-cost load balancing for routes with metrics no more than 3X the metric of the best route
• Do not use wildcard masks in the EIGRP configuration on routers R1, R2, and R3
• Just for fun (no other reason), use a wildcard mask when enabling EIGRP on R4, such that if additional interfaces were added to R4 in the future, those interfaces would not automatically be enabled for EIGRP
• Assume all IPv4 addresses implied in the figure are configured (per the initial configurations shown here)
• Assume all interfaces in the figure are up and working
• Use all defaults unless otherwise required to accomplish the lab requirements

Figure 1: Router Diamond with IP Subnets

Initial Configuration

Examples 1, 2, 3 and 4 show the beginning configuration state of R1, R2, R3 and R4.

Example 1: R1 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 10.10.10.1 255.255.255.0
!
interface GigabitEthernet0/2
 no shutdown 
 ip address 20.20.20.1 255.255.255.0
!
interface GigabitEthernet0/3
 no shutdown 
 ip address 192.168.1.1 255.255.255.0

Example 2: R2 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 30.30.30.1 255.255.255.0
!
interface GigabitEthernet0/2
 no shutdown 
 ip address 10.10.10.2 255.255.255.0
!
interface GigabitEthernet0/3
 no shutdown 
 ip address 40.40.40.1 255.255.255.0

Example 3: R3 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 20.20.20.2 255.255.255.0
!
interface GigabitEthernet0/2
 no shutdown 
 ip address 30.30.30.2 255.255.255.0
!
interface GigabitEthernet0/3
 no shutdown 
 ip address 50.50.50.1 255.255.255.0
 speed 100

Example 4: R4 Config

interface GigabitEthernet0/1
 no shutdown
 ip address 40.40.40.2 255.255.255.0
!
interface GigabitEthernet0/2
 no shutdown 
 ip address 50.50.50.2 255.255.255.0
 speed 100
!
interface GigabitEthernet0/3
 no shutdown 
 ip address 192.168.2.1 255.255.255.0

Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear, or other tools, configure the lab with those tools.

If you do try this lab beyond just writing the answers on paper or in a text editor, you have a great advantage for finding the actual metrics calculated for the alternate routes. I would suggest using the show ip eigrp topology command on R4, look at the entry for subnet 192.168.1.0/24, and find the metric for the successor (the best route) and the feasible successor (the next-best route). Then you can decide whether your configuration should or should not be correct for allowing both these routes into R4’s routing table. Ultimately, seeing two routes to 192.168.1.0/24 on R4 (with the show ip route command) proves the lab worked as intended.