# Implementing Single-Area OSPF
### **Guide**
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.
Topology
### **Step 1: Setup the Network in Packet Tracer**
1. Open **GNS3**.
2. Add **three routers** (R1, R2, R3).
3. Add **three PCs** (PC1, PC2, PC3) for testing connectivity.
4. 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
```
#### **On R3:**
```
R3#configure terminal
R3(config)#interface fastEthernet 0/0
R3(config-if)#ip address 192.168.13.2 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip address 192.168.23.2 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface loopback 0
R3(config-if)#ip address 3.3.3.3 255.255.255.255
R3(config-if)#exit
R3(config)#router ospf 1
R3(config-router)#network 192.168.23.0 0.0.0.3 area 0
R3(config-router)#network 192.168.13.0 0.0.0.3 area 0
R3(config-router)#network 3.3.3.3 0.0.0.0 area 0
```
### **Step 3: Verify OSPF Configuration**
#### **Checking OSPF Neighbors**
Run the following command on each router:
* R1
```
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#
```
### Keywords
`OSPF`, `single-area OSPF`, `OSPF configuration`, `OSPF adjacency`, `OSPF routing`, `Cisco OSPF`, `OSPF area 0`, `OSPF neighbor`, `OSPF cost`, `OSPF passive interface`, `OSPF router ID`, `OSPF topology`, `OSPF verification`, `OSPF troubleshooting`, `OSPF commands`, `OSPF network types`, `OSPF protocol`, `OSPF metrics`, `OSPF Packet Tracer`, `OSPF route table`, `سیسکو`
