OSPF configuration in Packet Tracer

Welcome ! This tutorial will give you an easy guide of configuring basic OSPF on a Cisco Router. But just before we get to configure, let’s have an overview of OSPF.

About OSPF

Open Shortest Path First(OSPF) is one of the dynamic routing protocols amongst others such as  EIGRP, BGP and and RIP. It is perhaps one of the most popular link state routing protocols. It is an open standard, so it can be run on routers from different vendors.

OSPF supports key features such as:

• IPv4 and IPv6 routing
• Classless routing
• Equal cost load balancing,
• Manual route summarization, etc.

OSPF has a default administrative distance of 110. It uses cost as the parameter for determining  route metric. It uses the multicast address of 224.0.0.5 and 224.0.0.6 for communication between OSPF-enabled neighbors

Routers running OSPF need to establish a neighbor relationship before exchanging routing updates. Each OSPF router runs the SFP algorithm to calculate the best routes and adds them to the routing table.

OSPF routers store routing and topology information in three tables.:

• Neighbor table-which stores information about OSPF neighbors.
• Topology table-stores topology structure of the network.
• Routing table-stores the best routes

OSPF neighborhood discovery

Routers running OSPF need to establish a neighbor relationship before exchanging routing updates. OSPF neighbors are dynamically discovered by sending Hello packets out each OSPF-enabled interface on a router. Hello packets are sent to the multicast address of 224.0.0.5.

OSPF areas

An area is simply a logical grouping of adjacent networks and routers. All routers in the same area have the same topology table and don’t know about routers in other areas. The main benefits of using areas in an OSPF network are:

• Routing tables on the routers are reduced.
• Routing updates are reduced.

Each area in an OSPF network must be connected to the backbone area ( also known as area 0 ). All routers inside an area must have the same area ID .

A router that has interfaces in more than one area (for example area 0 and area 1) is known as an Area Border Router (ABR).  A router that connects an OSPF network to other routing networks (for example, to an EIGRP network) is called an Autonomous System Border Router (ASBR).

For now we’ll configure basic OSPF. On to it then!

Basic  OSPF configuration.

1. Build the network topology.

2.Configure IP addresses on PCs and router interfaces.

Router 1

R1(config)#int fa 0/0
R1(config-if)#ip add 10.0.0.1 255.0.0.0
R1(config-if)#no shut
R1(config-if)#
R1(config-if)#int serial 0/0/0
R1(config-if)#ip add 20.0.0.1 255.0.0.0
R1(config-if)#no shut

Router 2

R2(config-if)#int fa0/0
R2(config-if)#ip add 30.0.0.1 255.0.0.0
R2(config-if)#no shut
R2(config-if)#
R2(config-if)#int serial0/0/0
R2(config-if)#ip address 20.0.0.2 255.0.0.0
R2(config-if)#no shut

Now do IP configurations for the PCs.

PC1  IP add 10.0.0.2  Subnet mask 255.0.0.0   Default gateway  10.0.0.1

PC2 IP add 30.0.0.2  Subnet mask 255.0.0.0    Default gateway   30.0.0.1

3. Configure OSPF on the routers.

The configuration is pretty simple and requires only two major steps:

1.  Enable OSPF on a router using the router ospf PROCESS_ID in the global configuration mode.

2.Define on which interfaces OSPF will run and what networks will be advertised using network IP_ADDRESS  WILCARD_MASK  AREA command in the OSPF configuration mode.

Note that the OSPF process ID doesn’t have to be the same on all routers in order for the routers to establish a neighbor relationship, but the area parameter has to be the same on all neighboring routers in order for the routers to become neighbors.

Router 1

R1(config)#
R1(config)#router ospf 1
R1(config-router)#network 10.0.0.0  0.255.255.255  area 0
R1(config-router)#network 20.0.0.0  0.255.255.255  area 0

Router 2

R2(config)#
R2(config)#router ospf  2
R2(config-router)#network 20.0.0.0  0.255.255.255 area 0
R2(config-router)#network 30.0.0.0  0.255.255.255 area 0

As you can see from the above picture,we just need to enable OSPF on the routers which then advertise the networks directly connected to each of them.

Have in mind: The OSPF process IDs used for the two routers  have been made optionally different but their area  numbers must be the same.

4. Verify OSPF configuration

First, let’s verify that the routers have established a neighbor relationship by typing the show ip ospf neighbor command on R1:

Next, to verify that R1 has learnt the route to 30.0.0.0/8 network, we’ll use  show ip route ospf command on R1:

Note that the letter O indicates OSPF routes.

Lastly, verify connectivity. Ping PC2 from PC1. Ping should be successful.

Other OSPF verification commands

• show ip ospf  neighbors detail

• show ip ospf database

• show ip ospf interface

All the best!

You may also like to read:

• How to configure Multi-Area OSPF in Packet Tracer
• EIGRP configuration in Packet Tracer
• OSPFv3 (OSPF for IPv6) configuration in Packet Tracer
• EIGRPv6 configuration in Packet Tracer

References:

• https://geek-university.com/ccna/configure-ospf/