The situation
You have three routers, three LANs and one internet link. Every interface is addressed and up, but no routes exist yet. Your job: make every LAN reach every other LAN, and send everything else to the ISP. Use the fewest routes that still work. You can do this on paper, or in Cisco Packet Tracer or a similar simulator.
- 1. Goal: PC1, PC2 and the server can all reach each other, and every LAN can reach the ISP.
| Router | Interface | Address | Connects to |
|---|---|---|---|
| R1 | Gi0/0 | 192.168.1.1/24 | LAN 1 (PC1 192.168.1.10) |
| Gi0/1 | 10.0.12.1/30 | R2 | |
| Gi0/2 | 203.0.113.2/30 | ISP (203.0.113.1) | |
| R2 | Gi0/0 | 10.0.12.2/30 | R1 |
| Gi0/1 | 10.0.23.1/30 | R3 | |
| Gi0/2 | 192.168.2.1/24 | LAN 2 (PC2 192.168.2.10) | |
| R3 | Gi0/0 | 10.0.23.2/30 | R2 |
| Gi0/1 | 192.168.3.1/24 | LAN 3 (server 192.168.3.10) |
Task 1: list what each router is missing
For each router, write down every network in the topology that is not directly connected to it. Those are the networks it needs a route for (or a default route that covers them).
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| Router | Not connected |
|---|---|
| R1 | 192.168.2.0/24, 192.168.3.0/24, 10.0.23.0/30, the internet |
| R2 | 192.168.1.0/24, 192.168.3.0/24, 203.0.113.0/30, the internet |
| R3 | 192.168.1.0/24, 192.168.2.0/24, 10.0.12.0/30, 203.0.113.0/30, the internet |
Task 2: decide where a default route fits
Which routers can use a default route to cover several of those networks at once? Which way should each default point?
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- R1: default to the ISP (203.0.113.1). The company networks still need specific routes, because they are not toward the ISP.
- R2: default to R1 (10.0.12.1). This covers 192.168.1.0/24, 203.0.113.0/30 and the internet in one line. It only needs a specific route for LAN 3.
- R3: a stub. One default to R2 (10.0.23.1) covers everything.
Task 3: write the configuration
Write the ip route commands for all three routers, then compare.
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⚠️ Based on Cisco IOS / IOS XE documentation, not run on a lab device.
ip route 192.168.2.0 255.255.255.0 10.0.12.2
ip route 192.168.3.0 255.255.255.0 10.0.12.2
ip route 10.0.23.0 255.255.255.252 10.0.12.2
ip route 0.0.0.0 0.0.0.0 203.0.113.1R1: three specific routes toward R2 and a default to the ISP.
ip route 192.168.3.0 255.255.255.0 10.0.23.2
ip route 0.0.0.0 0.0.0.0 10.0.12.1R2: one specific route toward R3; everything else toward R1.
ip route 0.0.0.0 0.0.0.0 10.0.23.1R3: one default route.
Seven routes in total. The route to 10.0.23.0/30 on R1 is optional for the LANs, but without it R1 would send traffic for R3's link address (10.0.23.2) to the ISP. You couldn't ping or SSH to that address from LAN 1.
Task 4: test it
From R1, ping the server using the LAN 1 interface as the source, then trace the path. These outputs are based on Cisco documentation, not run on a lab device.
R1#ping 192.168.3.10 source GigabitEthernet0/0 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.3.10, timeout is 2 seconds: Packet sent with a source address of 192.168.1.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/3 ms
R1#traceroute 192.168.3.10 source GigabitEthernet0/0 Type escape sequence to abort. Tracing the route to 192.168.3.10 VRF info: (vrf in name/id, vrf out name/id) 1 10.0.12.2 1 msec 1 msec 0 msec 2 10.0.23.2 1 msec 1 msec 1 msec 3 192.168.3.10 2 msec 1 msec 1 msec
What if the first ping shows .!!!! ?
A single dot followed by successes is normal on a fresh network. The first packet was dropped while a router waited for an ARP reply to learn the next MAC address. Run the ping again and it should show !!!!!.
Bonus: internet access
PC2 pings 198.51.100.80. The packet leaves R1 toward the ISP, but no reply ever comes back. The routes are correct. Why?
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PC2's address, 192.168.2.10, is private. Internet routers don't carry routes to private ranges, so the reply has nowhere to go. R1 needs NAT to replace the private source with its public address 203.0.113.2. The NAT course covers how to set that up.