Answers

To find the lowest IP addresses, calculate the subnet ID, and then add 1 in the 4^th octet. I won’t go into how to find the subnet ID – that’s all over the books – but I’ll entertain specific questions if you have them. Then you just need to configure the IP addresses using the ip address address mask interface subcommand, as shown here.

Figure 1: Three Router Topology w/ Sample IP Addresses for Calculations

interface GigabitEthernet0/0
 no shutdown
 ip address 10.1.87.1 255.255.255.128
!
interface GigabitEthernet0/2/0
 no shutdown
 ip address 10.1.1.129 255.255.255.128
!
interface GigabitEthernet0/3/0
 no shutdown
 ip address 10.1.88.129 255.255.255.128

Example 1: R1 Config

interface GigabitEthernet0/0
 no shutdown
 ip address 172.16.2.1 255.255.254.0
!
interface GigabitEthernet0/1/0
 no shutdown
 ip address 10.1.1.130 255.255.255.128
!
interface GigabitEthernet0/3/0
 no shutdown
 ip address 172.16.0.1 255.255.254.0

Example 2: R2 Config

interface GigabitEthernet0/0
 no shutdown
 ip address 10.1.89.1 255.255.255.128
!
interface GigabitEthernet0/1/0
 no shutdown
 ip address 10.1.88.130 255.255.255.128
!
interface GigabitEthernet0/2/0
 no shutdown
 ip address 172.16.0.2 255.255.254.0

Example 3: R3 Config

Before configuring any commands for this lab, you needed to plan your IP addresses. In this case, the figure shows the subnet IDs, and you have to find the numerically lowest address(es) to use for the routers. That makes the math pretty easy – just add one to the 4th octet of the subnet ID to get the numerically lowest IP address in the subnet.

Once planned, configuring the IP addresses require one command per interface: the ip address address mask command, with the address and dotted-decimal mask used as the two parameters.

Finally, for this lab, you should have considered adding the no shutdown command to each interface. The lab states that the routers have no configuration other than the hostname command. As a result, the interfaces may be in an administratively disabled (shutdown) state to begin the lab. The no shutdown command would then change the state to administratively enabled.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

1. From the console of each router, verify the router’s IP addresses:
   1. Use the show ip interface command to verify the address and mask.
   2. Use the show ip interfaces brief command to verify the addresses.
   3. Use the ping command against the addresses expected to be configured on the local router; all pings should work.
2. The initial configuration does not include any routing protocol configuration, so you should see only connected and local routes in the routing tables unless you chose to add routing protocol configuration. You can verify those routes as compared to the figure. On each router:
   1. Use the show ip route command. Look for three connected routes, one for each router interface per the figure.
3. Use ping tests to ping the router IP addresses on the opposite end of each WAN link:
   1. From router R1, ping the R2 WAN IP address on their shared link and then the R3 WAN IP address on their shared link.
   2. Likewise, from router R2, ping R1’s WAN IP address and R3’s WAN IP address.
   3. Finally, from R3, ping R1’s and R2’s WAN IP addresses on the respective shared links.