EIGRP Enabler #1 – Answers

The first EIGRP Enabler exercise asked you to configure four routers, specifically to add the EIGRP network commands. This post lists the answers – simple enough.

The Topology

It is probably best to have the exercise post on the screen at the same time you look at this post. If not, for convenience, here’s a copy of the topology figure for this exercise.

EIGRP Enabler Topology

The Answers

I promised short, sweet, and to the point. Here are the answers!

router eigrp 1
 network 172.17.0.0

Answers: Router R1

router eigrp 1
 network 172.17.4.0 0.0.3.255
 network 172.17.99.0 0.0.0.3

Answers: Router R2

router eigrp 1
 network 172.17.8.3 0.0.0.0
 network 172.17.99.43 0.0.0.0

Answers: Router R3

router eigrp 1
 network 172.17.16.0 0.0.15.255
 network 172.17.99.80 0.0.0.15

Answers: Router R4

Comments and Explanations – R1

For R1, you needed to start by identifying the various classful networks (class A, B, or C networks) used on the interfaces. Then, to configure one network command to match each classful network, just configure the network command with that classful network number, and with no wildcard mask. Simple enough.

In this case, R1 has five interfaces with IPv4 addresses, all of which are part of class B network 172.17.0.0. Therefore, R1 needs only a single command: network 172.17.0.0.

Comments and Explanations – R2

R2 requires the most thought. To get these commands totally correct per the requirements, you must:

1. Begin with each interface’s IP address and subnet mask
2. Calculate the subnet ID, which will be the first parameter in the network command
3. Invert the subnet mask, which will be the second parameter in the network command

Because of how the exercise is worded, you should have two network commands, one matching the subnet off each of the interfaces on R2.

Comments and Explanations – R3

R3 requires the least thought of the three requirements. To match a single IP address, use wildcard mask of 0.0.0.0. For the number preceding the wildcard mask in the network command, use the interface IP address. The result: each network command matches an interface IP address, and only that address, enabling EIGRP on that interface.

In this case, with two interfaces with IPv4 addresses, R3 needs two network commands, as shown in Example 3.

Comments and Explanations – R4

The requirements ask us to use the same logic to match each subnet as we used with router R2. The only twist here is that the subnet ID on one interface isn’t quite as obvious, and if you got in a hurry, you might have configured an incorrect subnet ID. Interface address 172.17.20.4, with mask 255.255.240.0, is in subnet 172.17.16.0, resulting in a correct command of network 172.17.16.0 0.0.15.255.