Answers

Figure 1: Simple Hub and Spoke Topology

Example 1: Core Config

Example 2: Branch1 Config

Example 3: Branch2 Config

Example 4: Branch3 Config

Commentary

When configuring static routes, it is important to ensure that you have all of the correct IPv4 subnet information, just a little difference in subnet mask can make a big difference in routing behavior.

For this lab you were tasked with configuring static routes to all remote subnets on all routers. The figure shows six different subnets. The Core router connects to three subnets, so it will need three static routes. Each branch router connects to two subnets, so each branch router will need static routes for four different subnets.

For Core, the three remote LAN subnets are 192.168.50.0/24, 192.168.51.0/24 and 192.168.52.0/24. Just to help us get our bearings, Table 1 lists the three destination subnets, with router Core’s outgoing interface and next-hop global unicast address:

Table 1: Data to Use in Router Core’s Static IPv4 Routes

The configuration, shown in Example 1, repeats the ip route command three times. Each time it lists the destination subnet per the first column of the table, and the next hop address per the last column. For example, the complete command to configure one static route would be ip route 192.168.50.0 255.255.255.0 10.100.200.2. (Note that the mask is in DDN format, not prefix format.)

For Branch1, before thinking about the configuration command, think about the topology in the figure. Branch1 has two connected subnets. Then, to send packets to the other four subnets, router Branch1 will always send the packets out its G0/1 interface to the Core router next. As a result, and as seen in Example 2, all four of Branch1’s routes list 10.100.200.1 as the next hop address. The four commands each list different destination subnets: 10.100.200.4/30, 192.168.51.0/24, 10.100.200.8/30, and 192.168.52.0/24.

Branch2 uses the same basic logic as router Branch1. Branch2 has two connected subnets, four remote subnets, and all four static routes for the remote subnets points to router Core next. In Branch2’s case, the next-hop address of the Core router is 10.100.200.5. As seen in Example 3, the four ip route commands list 10.100.200.5 as the next-hop address, with subnets 10.100.200.0/30, 192.168.50.0/24, 10.100.200.8/30, and 192.168.52.0/24 in the four different commands.

As you probably guess by now, Branch3 uses the same basic logic as routers Branch1 and Branch2. Branch3 has two connected subnets, four remote subnets, and all four static routes for the remote subnets points to router Core next. In Branch3’s case, the next-hop address of the Core router is 10.100.200.9. As seen in Example 4, the four ip route commands list 10.100.200.9 as the next-hop address, with subnets 10.100.200.0/30, 192.168.50.0/24, 10.100.200.4/30, and 192.168.51.0/24.

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. 

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 implement the entire network in a lab, you can test the solution by going to each PC and pinging the other three. All the pings should work. Additionally, you could use an extended ping command to ping from each router’s G0/2 interface to each other router’s G0/2 interface IP address. For instance, from R1, the command ping 192.168.52.1 source 192.168.50.1 would test the forward route to R4’s G0/2 subnet, and the reverse route back to R1’s G0/2 subnet. Finally, issuing a traceroute command from any of the PCs in the lab would let you see the path through the network.