OSPF Routing Protocol Configuration (Single Area and Multi-Area OSPF)

 Unlike other routing protocols, OSPF does not carry data via a transport protocol (UDP, TCP). Instead, OSPF forms IP datagrams directly, packaging them using protocol number 89 for the IP Protocol field.  It is an IGP means an interior gateway routing protocol that is used for communication in the same autonomous system. OSPF is the most loved protocol in the industry due to its useful features. From the calculation of the routes in the network, it uses an algorithm called Dijkstra Algorithm which calculates the shortest path from source to destination. Initially, all OSPF routers gather the information of their connected networks which is called link state database LSDB, then they share this information with other OSPF routers, and then the information is populated in the whole network. After gathering information the routers start calculating their shortest path to the destination, this calculation is based on the total cost through an interface.

Based on the configuration type, OSPF can be used in a single-area network or a multi-area network.

·         Single Area OSPF

Here we have only one area for all routers and that is Area 0 which is also called the backbone area. We use single area OSPF for comparatively small networks.

·         Multi Area OSPF

Large networks are divided into different areas to reduce the number of link state advertisements and other OSPF overhead traffic. This traffic is confined to the same area and not shared with other areas.

 

This division of a network into areas give us a concept of different type of routers based on their location in the network. Below are the different router types of an OSPF network.

Router types

·         Internal router (IR): Router located in a specific area.

·         Area border router (ABR): This router has one leg in Area 0 and another leg in any other Area.

·         Backbone router (BR): Router Area 0 is called backbone router,

·         Autonomous system boundary router (ASBR): The boundary router after which a different autonomous system with a different routing protocol starts

Router id

The unique ID for the Router in the OSPF topology is known as the Router ID. The router ID is a 32-bit IP address assigned to each OSPF router.OSPF router ID should not be changed once the neighborship establishes because changing the router ID resets the OSPF Process and neighboring routes get reset.

·         Router ID can be configured manually.

·         If no router-id is configured, the router makes the highest loopback IP as the router ID.

·         If no loopback IP is configured, a router selects the highest active interface as the router ID.

We can configure router ID using the below commands

R1>enable

R1#configure terminal

R1(config)#router ospf 100

R1(config-router)#router-id 10.10.10.10

R1(config-router)#exit

R1(config)#exit

 

Router Attributes

·         Designated router (DR): A router that is the main centralized router that controls an area.

·         Backup designated router (BDR): It acts as an assistant of the DR routers and acts as DR in case of failure of the DR router.

OSPF neighborship establishes after exchanging certain messages that are called Link state advertisements. Below is the message type that OSPF routers exchange with each other.

OSPF messages

·         Hello -> used by a router to discover neighbor routers and establish neighbor adjacency

·         Database Description (DBD)

·         Link State Request (LSR)

·         Link State Update (LSU)

·         Link State Acknowledgment (LSAck)

Single Area OSPF Configuration 

In single area OSPF, we have the same Area for all networks and the area is normally Area 0. Here we are again using the same topology now configured with the OSPF routing protocol.

Below are the configuration steps for the OSPF routing protocol.

R1

The connected routes of R1 are advertised in OSPF process 1 and Area 0.

R1#conf t

R1(config)#router ospf 1

R1(config-router)# network 10.10.10.8 0.0.0.3 area 0

R1(config-router)# network 192.168.10.0 0.0.0.255 area 0

R1(config-router)#exit

R2:

For successful neighborship between the two routers the Process ID, Area number, and the point-to-point network should be correctly configured.

R2#conf t

R2(config)#router ospf 1

R2(config-router)# network 10.10.10.8 0.0.0.3 area 0

R2(config-router)# network 192.168.20.0 0.0.0.255 area 0

R2(config-router)#!

 

After the configuration of the above steps on both of the routers, a successful neighborship is established between the routers, and this neighborship can be checked with the below command.

As the neighborship has been established, now the routers can successfully share their network information with each other in the form of the routing table.

Router R1 is learning the network connected to R2 through OSPF protocol and R2 should learn the networks connected to R2.

The information of router ID, subnets, Area, and all OSPF advertised networks can be seen with the below command.

The status cost and states of OSPF at the interface level can be queried with the below command.

Multi-Area OSPF Configuration 

In multi-area OSPF as the name indicates, we have multiple areas at least two. The main requirement of Multi Area OSPF is that all other areas should be connected to Area 0 to exchange routing information.

Let’s take the below topology where we have two different areas (Area 0 and Area1 ) communicating with each other. The routers that are having one leg in the other area are called ABR or Area border routers. In our topology, ABR_0 and Rarea1_1 are the area border routers.

Let’s see the configuration of the Area border routers which enable the communication between the routers.

RArea1_1

RArea1_1#conf t

RArea1_1(config)#router ospf 1

RArea1_1(config-router)# network 11.11.11.4 0.0.0.3 area 0

RArea1_1(config-router)# network 11.11.11.8 0.0.0.3 area 1

RArea1_1(config-router)#!

 

On the router RArea1_1, we have configured the above networks according to the location of their networks. One leg is connected to Area0 and the other is located in Area1.

ABR_0

The below configuration has been done of the Area 0 ABR routers to establish neighborship relation with the other ABR of Area 1

ABR_0#conf t

ABR_0(config)#

ABR_0(config)#router ospf 1

ABR_0(config-router)# network 10.10.10.8 0.0.0.3 area 0

ABR_0(config-router)# network 192.168.20.0 0.0.0.255 area 0

ABR_0(config-router)# network 11.11.11.4 0.0.0.3 area 0

Each ABR has established two neighborship relations, one with its local router and the other with the other Area, Area Border router.

Routing Table of Multi-Area OSPF

ABR_0

The routes with “O IA” are the inter-area routes, this router is getting these routes from a different area and that is Area 1.

RArea1_2

The router RArea1_2 is learning below “O IA” routes from Area 0 via its area border router

Packet Tracer file with configuration logs and script is available and may be provided upon request