Implementing Multiple OSPF Areas

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Objective

In this guide, we will configure multiple OSPF areas using Cisco routers in a practical scenario. You will learn how to:

  • Configure OSPF in a multi-area environment.

  • Define Area Border Routers (ABRs).

  • Verify OSPF neighbor relationships and route advertisements.

Scenario Overview

Imagine a company with three branches, each connected through an OSPF network:

  • Area 0 (Backbone Area): The core of the network, connecting all areas.

  • Area 1 (Branch 1): A separate area for one branch.

  • Area 2 (Branch 2): Another separate area for another branch.

Each branch should communicate through Area 0.

🖥️ Step 1: Network Topology

We will use five routers (R1, R2, R3, R4, R5) in this topology:

Topology

  • R1 is in Area 0 (Backbone).

  • R2 is an ABR (Area Border Router), connecting Area 0 and Area 1.

  • R3 is another ABR, connecting Area 0 and Area 2.

  • R4 belongs to Area 1.

  • R5 belongs to Area 2.

🛠️ Step 2: IP Addressing Scheme

Assign IP addresses as follows:

Device
Interface
IP Address
Area

R1

s3/0

192.168.0.1/30

Area 0

R2

s3/0

192.168.0.2/30

Area 0

R2

s3/1

192.168.1.1/30

Area 1

R4

s3/1

192.168.1.2/30

Area 1

R1

s3/1

192.168.2.1/30

Area 0

R3

s3/1

192.168.2.2/30

Area 0

R3

s3/2

192.168.3.1/30

Area 2

R5

s3/2

192.168.3.2/30

Area 2

⚙️ Step 3: Basic Configuring Each Router

📝 R1 

R1#configure terminal
R1(config)#interface serial 3/0
R1(config-if)#ip address 192.168.0.1 255.255.255.252
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)#do wr mem

📝 R2

R2#configure terminal
R2(config)#interface serial 3/0
R2(config-if)#ip address 192.168.0.2 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 3/1
R2(config-if)#ip address 192.168.1.1 255.255.255.252
R2(config-if)#no shutdown
R2(config-if)#do wr mem

📝 R3

R3#configure terminal
R3(config)#interface serial 3/1
R3(config-if)#ip address 192.168.2.2 255.255.255.252
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface serial 3/2
R3(config-if)#ip address 192.168.3.1 255.255.255.252
R3(config-if)#no shutdown                                                                                        changed state to up
R3(config-if)#do wr mem

📝 R4

R4#configure terminal
R4(config)#interface serial 3/1
R4(config-if)#ip address 192.168.1.2 255.255.255.252
R4(config-if)#no shutdown
R4(config-if)#do wr mem

📝 R5

R5#configure terminal
R5(config)#interface serial 3/2
R5(config-if)#ip address 192.168.3.2 255.255.255.252
R5(config-if)#no shutdown
R5(config-if)#do wr mem

⚙️ Step 4: Configuring OSPF on Each Router

Now, configure OSPF on each router.

📝 R1 (Backbone Router in Area 0)

R1(config)#router ospf 1
R1(config-router)#network 192.168.0.0 0.0.0.3 area 0
R1(config-router)#network 192.168.2.0 0.0.0.3 area 0

📝 R2 (ABR: Area 0 & Area 1)

R2(config)#router ospf 1
R2(config-router)#network 192.168.0.0 0.0.0.3 area 0
R2(config-router)#network 192.168.1.0 0.0.0.3 area 1

📝 R3 (ABR: Area 0 & Area 2)

R3(config)#router ospf 1
R3(config-router)#network 192.168.2.0 0.0.0.3 area 0
R3(config-router)#network 192.168.3.0 0.0.0.3 area 2

📝 R4 (Area 1 Router)

R4(config)#router ospf 1
R4(config-router)#network 192.168.1.0 0.0.0.3 area 1

📝 R5 (Area 2 Router)

R5(config)#router ospf 1
R5(config-router)#network 192.168.3.0 0.0.0.3 area 2

🔍 Step 5: Verify OSPF Configuration

Now, let's check if OSPF is working properly.

🔎 Verify OSPF Neighbor Relationships

On R1:

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.3.1       0   FULL/  -        00:00:39    192.168.2.2     Serial3/1
192.168.1.1       0   FULL/  -        00:00:33    192.168.0.2     Serial3/0

Output should show R2 and R3 as neighbors.

On R2:

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.2.1       0   FULL/  -        00:00:39    192.168.0.1     Serial3/0
192.168.1.2       0   FULL/  -        00:00:37    192.168.1.2     Serial3/1

Should list R1 and R4.

On R3:

/R3#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.2.1       0   FULL/  -        00:00:33    192.168.2.1     Serial3/1
192.168.3.2       0   FULL/  -        00:00:38    192.168.3.2     Serial3/2

Should list R1 and R5.

🔎 Verify OSPF Routing Table

Check the routing table on 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

      192.168.1.0/30 is subnetted, 1 subnets
O IA     192.168.1.0 [110/128] via 192.168.0.2, 00:07:53, Serial3/0
      192.168.3.0/30 is subnetted, 1 subnets
O IA     192.168.3.0 [110/128] via 192.168.2.2, 00:06:32, Serial3/1

It should display routes learned from Area 1 and Area 2.

🛠️ Step 6: Testing Connectivity

Use ping to ensure connectivity between different areas.

R4#ping 192.168.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.0.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/21/44 ms
R4#ping 192.168.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/27/48 ms
R4#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 = 52/66/96 ms
R4#

📌 Key Takeaways

  • OSPF Area 0 (Backbone Area) is mandatory and connects all areas.

  • Area Border Routers (ABRs) are needed to connect different areas.

  • Use show ip ospf neighbor to verify neighbor relationships.

  • Use show ip route ospf to verify routes learned via OSPF.

  • Testing with ping and traceroute ensures correct connectivity.

🛠️ Bonus: OSPF LSA Types in Multi-Area OSPF

  • Type 1 (Router LSA): Sent by every router within an area.

  • Type 2 (Network LSA): Sent by DR (Designated Router).

  • Type 3 (Summary LSA): Sent by ABRs to summarize routes between areas.

  • Type 4 (ASBR Summary LSA): Used when an ASBR exists.

  • Type 5 (External LSA): Used for external routes (e.g., redistribution).

Keywords

OSPF, multiple areas, Area 0, ABR, backbone area, OSPF configuration, router, network, routing table, neighbor relationships, OSPF routing, area border router, LSR, LSA types, OSPF neighbor, OSPF area types, OSPF area boundary, area 1, area 2, OSPF design, IP address configuration, سیسکو

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