As a Cisco CCNA Instructor, I will guide you through the process of implementing Single-Area OSPF (Open Shortest Path First) with a practical hands-on approach using Cisco Packet Tracer or real hardware.
Scenario:
Imagine you are a network engineer for a small company with three branch offices. You need to configure OSPF in a single area (area 0) to ensure efficient routing between the offices.
Network Topology:
We have 3 routers (R1, R2, and R3) connected in a triangular topology.
R1 is connected to R2 via the network 192.168.12.0/30
R2 is connected to R3 via the network 192.168.23.0/30
R1 is connected to R3 via the network 192.168.13.0/30
Each router has a loopback interface representing its unique router ID.
Step 1: Setup the Network in Packet Tracer
Open GNS3.
Add three routers (R1, R2, R3).
Add three PCs (PC1, PC2, PC3) for testing connectivity.
Connect them as follows:
R1 ↔ R2
R2 ↔ R3
R1 ↔ R3
Each router has a loopback interface (for OSPF Router-ID).
Step 2: Configure Basic Settings
Apply basic configurations on all routers.
On R1:
R1#configure terminal
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip address 192.168.12.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastEthernet 1/0
R1(config-if)#ip address 192.168.13.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface loopback 0 changed state to up
R1(config-if)#ip address 1.1.1.1 255.255.255.255
R1(config-if)#exit
R1(config)#router ospf 1
R1(config-router)#router-id 1.1.1.1
R1(config-router)#network 192.168.12.0 0.0.0.3 area 0
R1(config-router)#network 192.168.13.0 0.0.0.3 area 0
R1(config-router)#network 1.1.1.1 0.0.0.0 area 0
On R2:
R2#configure terminal
R2(config)#interface fastEthernet 0/0
R2(config-if)#ip address 192.168.12.2 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface fastEthernet 1/0
R2(config-if)#ip address 192.168.23.1 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface loopback 0 changed state to up
R2(config-if)#ip address 2.2.2.2 255.255.255.0
R2(config-if)#exit
R2(config)#router ospf 1
R2(config-router)#router-id 2.2.2.2
R2(config-router)#network 192.168.12.0 0.0.0.3 area 0
R2(config-router)#network 192.168.23.0 0.0.0.3 area 0
R2(config-router)#network 2.2.2.2 0.0.0.0 area 0
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 1 FULL/BDR 00:00:39 192.168.13.2 FastEthernet1/0
2.2.2.2 1 FULL/BDR 00:00:32 192.168.12.2 FastEthernet0/0
R1#
R2
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 1 FULL/BDR 00:00:35 192.168.23.2 FastEthernet1/0
1.1.1.1 1 FULL/DR 00:00:34 192.168.12.1 FastEthernet0/0
R2#
R3
R3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:33 192.168.13.1 FastEthernet0/0
2.2.2.2 1 FULL/DR 00:00:31 192.168.23.1 FastEthernet1/0
R3#
Checking OSPF Routing Table
R1#show ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/2] via 192.168.12.2, 00:07:19, FastEthernet0/0
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/2] via 192.168.13.2, 00:03:54, FastEthernet1/0
192.168.23.0/30 is subnetted, 1 subnets
O 192.168.23.0 [110/2] via 192.168.13.2, 00:03:54, FastEthernet1/0
[110/2] via 192.168.12.2, 00:07:29, FastEthernet0/0
R1#
Each router should have OSPF routes learned from other routers.
Testing Connectivity
Ping from R1 to R2,3's loopback:
R1#ping 2.2.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/22/36 ms
R1#ping 3.3.3.3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/12/32 ms
R1#
