Author: Andrei • Date: 2025-10-27

Build a small enterprise network with four routers and enable OSPF area 0 so that East-PC can reach West-PC through the DataCenter router. Border is included as an external neighbor on the same area for completeness.

• Border-Router1 Gi0/0 ↔ DataCenter-Router3 Gi0/2
• DataCenter-Router3 Gi0/0 ↔ East-Router2 Gi0/0
• DataCenter-Router3 Gi0/1 ↔ West-Router4 Gi0/0
• East-Router2 Gi0/1 ↔ East-Switch Fa0/1 ↔ East-PC Fa0
• West-Router4 Gi0/1 ↔ West-Switch Fa0/1 ↔ West-PC Fa0

<tr><td>DataCenter ↔ East</td><td>East-Router2</td><td>Gi0/0</td><td>10.12.1.21</td><td>255.255.255.252</td></tr>
<tr><td>DataCenter ↔ East</td><td>DataCenter-Router3</td><td>Gi0/0</td><td>10.12.1.22</td><td>255.255.255.252</td></tr>

<tr><td>DataCenter ↔ West</td><td>West-Router4</td><td>Gi0/0</td><td>10.12.1.33</td><td>255.255.255.252</td></tr>
<tr><td>DataCenter ↔ West</td><td>DataCenter-Router3</td><td>Gi0/1</td><td>10.12.1.34</td><td>255.255.255.252</td></tr>

<tr><td>East LAN</td><td>East-Router2</td><td>Gi0/1</td><td>10.12.20.1</td><td>255.255.255.0</td></tr>
<tr><td>West LAN</td><td>West-Router4</td><td>Gi0/1</td><td>10.12.40.1</td><td>255.255.255.0</td></tr>

• East-PC IP 10.12.20.10 • Mask 255.255.255.0 • GW 10.12.20.1
• West-PC IP 10.12.40.10 • Mask 255.255.255.0 • GW 10.12.40.1

enable
conf t
hostname Border-Router1
interface g0/0
 ip address 10.12.1.1 255.255.255.248
 no shutdown
!
router ospf 1
 network 10.12.1.0 0.0.0.7 area 0
end
write memory

enable
conf t
hostname DataCenter-Router3
interface g0/0
 ip address 10.12.1.22 255.255.255.252
 no shutdown
interface g0/1
 ip address 10.12.1.34 255.255.255.252
 no shutdown
interface g0/2
 ip address 10.12.1.2 255.255.255.248
 no shutdown
!
router ospf 1
 network 10.12.1.20 0.0.0.3 area 0
 network 10.12.1.32 0.0.0.3 area 0
 network 10.12.1.0  0.0.0.7 area 0
end
write memory

enable
conf t
hostname East-Router2
interface g0/0
 ip address 10.12.1.21 255.255.255.252
 no shutdown
interface g0/1
 ip address 10.12.20.1 255.255.255.0
 no shutdown
!
router ospf 1
 network 10.12.1.20 0.0.0.3 area 0
 network 10.12.20.0 0.0.0.255 area 0
end
write memory

enable
conf t
hostname West-Router4
interface g0/0
 ip address 10.12.1.33 255.255.255.252
 no shutdown
interface g0/1
 ip address 10.12.40.1 255.255.255.0
 no shutdown
!
router ospf 1
 network 10.12.1.32 0.0.0.3 area 0
 network 10.12.40.0 0.0.0.255 area 0
end
write memory

show ip ospf neighbor

Expected on DataCenter-Router3: neighbors for East (on Gi0/0) and West (on Gi0/1). Expected on East and West: one neighbor each to DataCenter. Border will also show as a neighbor on Gi0/2.

show ip route

Expected entries marked with O for:

• On East: 10.12.40.0/24 via 10.12.1.22
• On West: 10.12.20.0/24 via 10.12.1.34
• On DataCenter: 10.12.20.0/24 via 10.12.1.21 and 10.12.40.0/24 via 10.12.1.33

East-PC> ping 10.12.40.10
West-PC> ping 10.12.20.10

Both tests should return replies.

1. Topology view after everything is green
2. East-PC ping to West-PC success
3. show ip route on DataCenter-Router3
4. show ip route on East-Router2
5. show ip route on West-Router4
6. show ip ospf neighbor on DataCenter-Router3

• If neighbors do not form, confirm both ends of a link are in the same area and the correct network is matched. Interface method is a quick fix:

  interface g0/0
   ip ospf 1 area 0
  

• If ping fails with host unreachable from a PC, check the PC gateway and the router LAN address.
• If a link shows up on one side and down on the other, verify the cable type is Copper Straight-Through and interfaces are no shutdown.
• Use show cdp neighbors to confirm which interfaces are actually connected.

I practiced building an OSPF area 0 design with point-to-point links that use small subnets for efficient addressing. I verified adjacencies, checked OSPF-learned routes, and validated end-to-end PC reachability. I also used CDP and interface checks to resolve neighbor issues fast when an interface or port did not match the cabling.

End of Tech Journal