OSPF Single Area Configuration

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Introduction

Open Shortest Path First (OSPF) is a link-state routing protocol used in medium to large networks due to its scalability and fast convergence. In this guide, we will configure Single-Area OSPF in GNS3, step by step, using a practical topology.

Step 1: Network Topology & Scenario

We will use three routers (R1, R2, and R3), all belonging to OSPF Area 0 (Backbone Area).

Network Diagram

R1 connects to R2 using 192.168.12.0/30
R2 connects to R3 using 192.168.23.0/30
All routers will participate in OSPF Area 0

Step 2: Setting Up GNS3

Open GNS3 and create a new project.
Drag and drop three routers (e.g., Cisco 7200 or C3725).
Connect the routers as shown in the diagram using Ethernet links.
Assign IP addresses to interfaces.

Step 3: Configuring IP Addresses

Router 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

Router 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)#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

Router R3

R3#configure terminal
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip address 192.168.23.2 255.255.255.252
R3(config-if)#no shutdown

Step 4: Configuring OSPF (Single Area 0)

Router R1

R1(config)#router ospf 1
R1(config-router)#network 192.168.12.0 0.0.0.3 area 0

Router R2

R2(config)#router ospf 1
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

Router R3

R3(config)#router ospf 1
R3(config-router)#network 192.168.23.0 0.0.0.3 area 0

Step 5: Verify OSPF Configuration

Check OSPF Neighbor Relationships

Run this command on all routers:

Router R1

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.23.1      1   FULL/DR         00:00:39    192.168.12.2    FastEthernet0/0

Router R2

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.23.2      1   FULL/BDR        00:00:30    192.168.23.2    FastEthernet1/0
192.168.12.1      1   FULL/BDR        00:00:32    192.168.12.1    FastEthernet0/0

Router R3

R3#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.23.1      1   FULL/DR         00:00:38    192.168.23.1    FastEthernet1/0

Check OSPF Routing Table

Run this command on all routers:

Router R1

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

      192.168.23.0/30 is subnetted, 1 subnets
O        192.168.23.0 [110/2] via 192.168.12.2, 00:07:02, FastEthernet0/0
R1#

Router R2

R2#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

R2#

Router R3

R3#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

      192.168.12.0/30 is subnetted, 1 subnets
O        192.168.12.0 [110/2] via 192.168.23.1, 00:08:09, FastEthernet1/0
R3#

Step 5: Testing Connectivity

Ping from R1 to R3 to test end-to-end reachability:

R1#ping 192.168.23.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.23.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/48/80 ms
R1#

A successful ping means that OSPF has correctly learned and propagated routes.

Keywords

OSPF, single area, configuration, GNS3, routing, Cisco, neighbor adjacency, link-state, area 0, SPF algorithm, LSAs, network topology, IP addressing, OSPF process, routing table, cost metric, hello packets, DR/BDR, route propagation, troubleshooting, CCNP, CCNA, سیسکو

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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)#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#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.23.2 1 FULL/BDR 00:00:30 192.168.23.2 FastEthernet1/0 192.168.12.1 1 FULL/BDR 00:00:32 192.168.12.1 FastEthernet0/0

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 192.168.23.0/30 is subnetted, 1 subnets O 192.168.23.0 [110/2] via 192.168.12.2, 00:07:02, FastEthernet0/0 R1#

R2#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 R2#

R3#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 192.168.12.0/30 is subnetted, 1 subnets O 192.168.12.0 [110/2] via 192.168.23.1, 00:08:09, FastEthernet1/0 R3#

R1#ping 192.168.23.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.23.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 20/48/80 ms R1#