OSPF Multi-Area Configuration Explained

OSPF uses areas to limit the routing information routers share. By default, it keeps all routers in area 0. If required, you can create additional areas. If you use more than one area, you must connect at least one router from each area to area 0. That router is known as an ABR (Area Border Router).

OSPF routers can share routing information in two formats: detailed and summarized. By default, they share routing information only in the detail format. Routing updates in a detailed format consume a significant amount of bandwidth and require more RAM and CPU cycles to store and process. An OSPF router needs detailed routing information only about the routes available within the area. It does not need detailed information about the routes available within another area. You can configure the ABR router to summarize the routing information before forwarding it to another OSPF area. You can summarize only the contiguous subnets.

OSPF multi-area configuration (Packet Tracer Example)

Create a practice LAB as shown in the following image.

Download the practice LAB with IP configuration.

LAB objectives:-

Create, design, and implement multiple OSPF areas
Run a router's interfaces in different OSPF areas
Summarize OSPF routes
Verify OSPF multi-area configuration

Enabling OSPF routing in multi-area

Create three OSPF areas. Since area 0 is compulsory, create it first. Later, expand the design and add two more OSPF areas. OSPF areas are interface-specific. You can configure a router's interfaces in different OSPF areas. This network has eight routers. R1 and R5 have four serial interfaces. Configure OSPF area 0 on R1's serial 0/0/0 and R5's serial 0/0/0 interfaces. Configure R1's remaining interfaces in OSPF area 1 and R5's remaining interfaces in OSPF area 2. Configure R2, R3, and R4 in OSPF area 1 and R6, R7, and R8 in OSPF area 2. The following image shows the layout of OSPF areas to implement on this network.

The following two commands enable OSPF routing on a router and add interfaces to it.

Router(config)# router ospf [process_ID]
Router(config-router)# network IP_network_# [wildcard_mask] area-id [area number]

The first command enables an OSPF process under the specified process ID. You can run multiple OSPF processes on a single router. OSPF uses the process ID to identify each process. The process ID is a numeric value. It can be any number from 1 to 65,535. It is locally significant.

The second command adds interfaces to the OSPF process. It matches the given IP address with the IP address of all interfaces. If an interface's IP address matches the given IP address, it enables the OSPF process on that interface and adds that interface to the specified OSPF area.

You can use the wildcard mask to match multiple interfaces with a single configuration line. A wildcard mask tells the router the part of the address it should match. It contains wildcard bits. The area ID is the area number you want the interface to be in. It can be an integer between 0 and 4294967295.

The following commands enable OSPF routing under process ID 1 on R1, add its serial 0/0/0 interface to OSPF area 0, and the remaining interfaces to area 1.

Router>enable
Router#configure terminal
Router(config)#router ospf 1
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0
Router(config-router)#network 172.168.0.0 0.0.255.255 area 1
Router(config-router)#exit
Router(config)#exit
Router#

Apart from the highlighted commands, the remaining commands are used only to enter and exit the relevant configuration mode. Let us understand the highlighted commands.

Router(config)#router ospf 1	This command enables an OSPF process, sets its ID to 1, and enters OSPF process configuration mode.
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0	This command checks the IP configuration of all interfaces to add the interface having the network number 192.168.1 in its IP address to OSPF area 0.
Router(config-router)#network 172.168.0.0 0.0.255.255 area 1	This command checks the IP configuration of all interfaces to add the interface having the network number 172.168 in its IP address to OSPF area 1.

Use the same commands to enable the OSPF process and add interfaces to their respective OSPF areas on the remaining routers.

R2

Router>enable
Router#configure terminal
Router(config)#router ospf 2
Router(config-router)#network 172.168.0.0 0.0.255.255 area 1
Router(config-router)#exit
Router(config)#exit
Router#

R3

Router>enable
Router#configure terminal
Router(config)#router ospf 3
Router(config-router)#network 172.168.0.0 0.0.255.255 area 1
Router(config-router)#exit
Router(config)#

R4

Router>enable
Router#configure terminal
Router(config)#router ospf 4
Router(config-router)#network 172.168.0.0 0.0.255.255 area 1
Router(config-router)#exit
Router(config)#exit
Router#

R5

Router>enable
Router#configure terminal
Router(config)#router ospf 5
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0
Router(config-router)#network 10.0.0.0 0.255.255.255 area 2
Router(config-router)#exit
Router(config)#exit
Router#

R6

Router>enable
Router#configure terminal
Router(config)#router ospf 6
Router(config-router)#network 10.0.0.0 0.255.255.255 area 2
Router(config-router)#exit
Router(config)#exit
Router#

R7

Router>enable
Router#configure terminal
Router(config)#router ospf 7
Router(config-router)#network 10.0.0.0 0.255.255.255 area 2
Router(config-router)#exit
Router(config)#exit
Router#

R8

Router>enable
Router#configure terminal
Router(config)#router ospf 8
Router(config-router)#network 10.0.0.0 0.255.255.255 area 2
Router(config-router)#exit
Router(config)#exit
Router#

Testing and verifying OSPF multi-area configuration

Use the show ip route ospf command to verify the OSPF multi-area configuration. If the output of this command shows routes for all subnets from all OSPF areas, it verifies the OSPF multi-area configuration. The following image shows the output of this command on R2.

You can run this command on all routers to verify the OSPF multi-area configuration.

Use the ping command to test connectivity between end devices. To print the path data packets take to reach the destination, use the tracert command. The following image shows the output of these commands on PC0.

You can run these commands on all end devices to test their connectivity with other devices.

Download the practice LAB with OSPF configuration.

Route summarization on ABR

Without route summarization, a multi-area configuration has no benefits over a single-area configuration. To take full advantage of multi-area configuration, you should summarize routes on ABR routers in the direction of the backbone area. Since area 0 routers inject routing information into other OSPF areas, they will receive only summarized information. You can summarize only contiguous subnets. In this example network, R1 and R5 are ABR (Area Border Routers) routers. You can summarize routes on these routers. R1 connects area 1 with area 0. Area 1 contains the following contiguous subnets.

172.168.0.0/30
172.168.0.4/30
172.168.0.8/30
172.168.1.0/24
172.168.2.0/24
172.168.3.0/24

All these subnets start with the network number 172.168. You can summarize these subnets in the network 172.168.0.0/16. The following OSPF configuration mode command summarizes routes.

area [area-id] range [address] [mask]

The 'area-id' is the area that contains networks to be summarized.
The 'address' is the network number of the summarized route.
The 'mask' is the subnet mask of the summarized route.

The following command summarizes routes on R1. R1 will forward the summarized route to area 0.

Router(config)#router ospf 1
Router(config-router)#area 1 range 172.168.0.0 255.255.0.0
Router(config-router)#exit

R5 connects area 2 with area 0. Area 2 has the following contiguous subnets.

10.0.0.0/30
10.0.0.4/30
10.0.0.8/30
10.1.1.0/24
10.1.2.0/24
10.1.3.0/24

You can summarize the above subnets in the subnet 10.0.0./8. The following command summarizes routes on R5. R5 will forward the summarized route to area 0.

Router(config)#router ospf 5
Router(config-router)#area 2 range 10.0.0.0 255.0.0.0
Router(config-router)#exit

After route summarization, routers in area 1 and area 2 will see only the summarized routes of each other. However, they will keep detailed information about the routes available within their respective OSPF areas.

Use the show ip route ospf command to verify the summarized routes. The following image shows the output of this command on R2.

Download the practice LAB with OSPF Route summarization.

This tutorial is part of the tutorial "OSPF Configuration and Concepts Explained.". Other parts of this tutorial are as follows:

Chapter 01   OSPF (Open Shortest Path First) Protocol
Chapter 02   RIP V/s OSPF | Differences between RIP and OSPF
Chapter 03   IGP, EGP, and Autonomous System Explained
Chapter 04   OSPF Features, Advantages, Disadvantages
Chapter 05   OSPF Fundamental Terminology Explained
Chapter 06   OSPF LSA Types and LSA Flooding Explained
Chapter 07   OSPF Area Types and Concept Explained
Chapter 08   OSPF Hello Protocol and Packets Explained
Chapter 09   OSPF RID (Router ID) Explained
Chapter 10   OSPF Neighborship Condition and Requirement
Chapter 11   OSPF DR BDR Selection Process Explained
Chapter 12   How OSPF Routers Build Adjacency Explained
Chapter 13  Shortest Path First (SPF) Algorithm Explained
Chapter 14   OSPF Single-Area Configuration Explained
Chapter 15   OSPF Stub area, Totally Stub area, NSSA, and Totally NSSA
Chapter 16   OSPF Virtual Links Explained
Chapter 17   OSPF Authentication Password and MD5 Explained
Chapter 18   OSPF Multi-Area Configuration Explained

Conclusion

OSPF supports multi-area configuration. If you design a multi-area network, you can summarize the routing information on ABR routers. After summarization, ABR routers will forward only the summarized routes to other OSPF areas.

Author Laxmi Goswami Updated on 2026-05-14