In this guide, you'll learn how to configure OSPF Multi-Area in GNS3step by step using a practical topology.
Step 1: Understanding OSPF Multi-Area
OSPF (Open Shortest Path First) is a link-state routing protocol that divides large networks into smaller, more manageable areas. This improves scalability and performance.
Backbone Area (Area 0) – The central area where all other areas must connect.
Regular Areas (Area 1, Area 2, etc.) – Non-backbone areas connected to Area 0.
ABR (Area Border Router) – Connects different OSPF areas.
ASBR (Autonomous System Boundary Router) – Connects OSPF to external networks.
Step 2: Network Topology
We'll configure OSPF Multi-Area using the following topology:
Step 3: Configuring OSPF Multi-Area in GNS3
Open GNS3 and Create the Topology
Drag and drop 5 Cisco routers (e.g., IOSv or 7200).
Connect interfaces as per the IP Addressing Scheme.
Assign IP addresses to each router interface.
Configuring R1 (ABR for Area 0 and Area 1)
R1#configure terminal
R1(config)#interface serial 3/0
R1(config-if)#ip address 192.168.1.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface serial 3/1
R1(config-if)#ip address 192.168.2.1 255.255.255.252
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#router ospf 1
R1(config-router)#network 192.168.1.0 0.0.0.3 area 0
R1(config-router)#network 192.168.2.0 0.0.0.3 area 1
Configuring R2 (ABR for Area 0 and Area 2)
R2#configure terminal
R2(config)#interface serial 3/1
R2(config-if)#ip address 192.168.3.1 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 3/0
R2(config-if)#ip address 192.168.1.2 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 3/2 changed state to down
R2(config-if)#ip address 192.168.4.1 255.255.255.252
R2(config-if)#no shutdown changed state to up
R2(config-if)#router ospf 1
R2(config-router)#network 192.168.1.0 0.0.0.3 area 0
R2(config-router)#network 192.168.3.0 0.0.0.3 area 2
R2(config-router)#network 192.168.4.0 0.0.0.3 area 2
Configuring R3 (Area 2)
R3#configure terminal
R3(config)#interface serial 3/1
R3(config-if)#ip address 192.168.3.2 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#router ospf 1
R3(config-router)#network 192.168.3.0 0.0.0.3 area 2
Configuring R4 (Area 1)
R4#configure terminal
R4(config)#interface serial 3/1
R4(config-if)#ip address 192.168.2.2 255.255.255.252
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#router ospf 1
R4(config-router)#network 192.168.2.0 0.0.0.3 area 1
Configuring R5 (Area 2)
R5#configure terminal
R5(config)#interface serial 3/2
R5(config-if)#ip address 192.168.4.2 255.255.255.252
R5(config-if)#no shutdown
R5(config-if)#exit
R5(config)#router ospf 1
R5(config-router)#network 192.168.4.0 0.0.0.3 area 2
Step 4: Verifying OSPF Multi-Area Configuration
Check OSPF Neighbor Adjacencies
On each router, run:
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.2.1 0 FULL/ - 00:00:39 192.168.1.1 Serial3/0
192.168.4.2 0 FULL/ - 00:00:35 192.168.4.2 Serial3/2
192.168.3.2 0 FULL/ - 00:00:37 192.168.3.2 Serial3/1
R2#
This should display adjacent routers.
Check OSPF Routing Table
R5#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
192.168.1.0/30 is subnetted, 1 subnets
O IA 192.168.1.0 [110/128] via 192.168.4.1, 00:06:45, Serial3/2
192.168.2.0/30 is subnetted, 1 subnets
O IA 192.168.2.0 [110/192] via 192.168.4.1, 00:06:45, Serial3/2
192.168.3.0/30 is subnetted, 1 subnets
O 192.168.3.0 [110/128] via 192.168.4.1, 00:06:45, Serial3/2
R5#
Ensure routes from different areas appear as Inter-Area (O IA).
Check OSPF Database
R2#show ip ospf database
OSPF Router with ID (192.168.4.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
192.168.2.1 192.168.2.1 543 0x80000004 0x0097EE 2
192.168.4.1 192.168.4.1 637 0x80000002 0x007312 2
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.2.0 192.168.2.1 534 0x80000001 0x001216
192.168.3.0 192.168.4.1 628 0x80000001 0x00F82C
192.168.4.0 192.168.4.1 442 0x80000001 0x00ED36
Router Link States (Area 2)
Link ID ADV Router Age Seq# Checksum Link count
192.168.3.2 192.168.3.2 632 0x80000001 0x00CAB6 2
192.168.4.1 192.168.4.1 442 0x80000005 0x000195 4
192.168.4.2 192.168.4.2 447 0x80000001 0x00DAA2 2
Summary Net Link States (Area 2)
Link ID ADV Router Age Seq# Checksum
192.168.1.0 192.168.4.1 638 0x80000001 0x000F18
192.168.2.0 192.168.4.1 533 0x80000001 0x00865F
R2#
Verify LSAs are correctly exchanged.
Step 5: Testing Connectivity
R1#ping 192.168.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/26/52 ms
R1#ping 192.168.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/9/20 ms
R1#ping 192.168.3.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/10/12 ms
R1#ping 192.168.3.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/34/48 ms
R1#ping 192.168.4.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/34/52 ms
R1#
If successful, the OSPF Multi-Area setup is correctly configured!
