OSPF Configuration Step by Step Guide

This tutorial explains how to configure OSPF Routing protocol step by step with practical example in packet tracer. Learn OSPF configuration commands, OSPF show commands, OSPF network configuration (Process ID, Network ID, Wild card mask and Area number) and OSPF routing in detail.

For demonstration we will use packet tracer network simulator software. You can use real Cisco devices or any other network simulator software for following this guide.

Create a topology as illustrate in following figure or download this pre-created topology.

Download OSPF Practice Topology

Initial IP Configuration

Assign IP address to PC

Double click PC0 and click Desktop menu item and click IP Configuration. Assign IP address 10.0.0.2/8 to PC0.

Repeat same process for Server0 and assign IP address 20.0.0.2/8.

Assign IP address to interfaces of routers

Double click Router0 and click CLI and press Enter key to access the command prompt of Router0.

Four interfaces FastEthernet0/0, FastEthernet0/1, Serial 0/0/0 and Serial0/0/1 of Router0 are used in this topology. By default interfaces on router are remain administratively down during the start up.

We need to configure IP address and other parameters on interfaces before we could actually use them for routing. Interface mode is used to assign the IP address and other parameters. Interface mode can be accessed from global configuration mode. Following commands are used to access the global configuration mode.

Router>enable
Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#

From global configuration mode we can enter in interface mode. From there we can configure the interface. Following commands will assign IP address on FastEthernet0/0 and FastEthernet0/1.

Router(config)#interface fastEthernet 0/0
Router(config-if)#ip address 10.0.0.1 255.0.0.0
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#interface fastEthernet 0/1
Router(config-if)#ip address 192.168.1.1 255.255.255.252
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#

interface fastEthernet 0/0 command is used to enter in interface mode.

ip address 10.0.0.1 255.0.0.0 command would assign IP address to interface.

no shutdown command would bring the interface up.

exit command is used to return in global configuration mode.

Serial interface needs two additional parameters clock rate and bandwidth. Every serial cable has two ends DTE and DCE. These parameters are always configured at DCE end.

We can use show controllers interface command from privilege mode to check the cable’s end.

Router#show controllers serial 0/0/0
Interface Serial0/0/0
Hardware is PowerQUICC MPC860
DCE V.35, clock rate 2000000
[Output omitted]

Fourth line of output confirms that DCE end of serial cable is attached. If you see DTE here instead of DCE skip these parameters.

Now we have necessary information let’s assign IP address to serial interfaces.

Router# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface serial 0/0/0
Router(config-if)#ip address 192.168.0.1  255.255.255.252
Router(config-if)#clock rate 64000
Router(config-if)#bandwidth 64
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#interface serial 0/0/1
Router(config-if)#ip address 192.168.2.1  255.255.255.252
Router(config-if)#no shutdown
Router(config-if)#exit

Router#configure terminal Command is used to enter in global configuration mode.

Router(config)#interface serial 0/0/0 Command is used to enter in interface mode.

Router(config-if)#ip address 192.168.0.1 255.255.255.252 Command assigns IP address to interface. For serial link we usually use IP address from /30 subnet.

Router(config-if)#clock rate 64000 In real life environment this parameter controls the data flow between serial links and need to be set at service provider’s end. In lab environment we need not to worry about this value. We can use any valid clock rate here.

Router(config-if)#bandwidth 64 Bandwidth works as an influencer. It is used to influence the metric calculation of OSPF or any other routing protocol which uses bandwidth parameter in route selection process. Serial interface has default bandwidth of 1544Kbps. To explain, how bandwidth influence route selection process we will configure (64Kbps) bandwidth on three serial DCE interfaces of our network; R0’s Se0/0/0, R1’s Se0/0/1 and R2’s Se0/0/0.

Router(config-if)#no shutdown Command brings interface up.

Router(config-if)#exit Command is used to return in global configuration mode.

We will use same commands to assign IP addresses on interfaces of remaining routers.

Router1

Router2

As I mention earlier, serial interface has a default bandwidth of 1544Kbps. If we don’t assign any custom bandwidth, router would use default bandwidth. To see this feature in action we will not assign bandwidth on remaining routers.

Router6

Router5

Router3

Router4

Great job we have finished our half journey. Now routers have information about the networks that they have on their own interfaces. Routers will not exchange this information between them on their own. We need to implement OSPF routing protocol that will insist them to share this information.

Configure OSPF routing protocol

Enabling OSPF is a two steps process:-

• Enable OSPF routing protocol from global configuration mode.
• Tell OSPF which interfaces we want to include.

For these steps following commands are used respectively.

Router(config)# router ospf process_ID
Router(config-router)# network IP_network_# [wild card mask] Area Number area number

Router(config)# router ospf process ID

This command will enable OSPF routing protocol in router. Process ID is a positive integer. We can use any number from 1 to 65,535. Process ID is locally significant. We can run multiple OSPF process on same router. Process ID is used to differentiate between them. Process ID need not to match on all routers.

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

Network command allows us to specify the interfaces which we want to include in OSPF process. This command accepts three arguments network number, wildcard mask and area number.

Network number

Network number is network ID. We can use any particular host IP address or network IP address. For example we can use 192.168.1.1 (host IP address) or we can use 192.168.1.0 (Network IP address). While targeting a specific interface usually we use host IP address (configured on that interface).

While targeting multiple interfaces, we use network IP address. So any interface that belongs to specified network ID will be selected.

Wildcard mask

Wildcard mask are used with network ID to filter the interfaces. Wildcard mask is different from subnet mask. Subnet mask is used to separate the network portion and host portion in IP address. While wildcard mask is used to match corresponding octet in network portion. Wildcard mask tells OSPF the part of network address that must be matched. Wildcard masks are explained with examples in access list tutorials of this category.

Key points

0 (Decimal – octet format) Wildcard mask indicates that corresponding octet in network address must be matched exactly.

255 (Decimal – octet format) Wildcard mask indicates that we don’t care about corresponding octet in network address.

For example

0 (Binary – bit format) Wildcard mask indicates that corresponding bit in network address must be matched exactly.

255 (Binary – bit format) Wildcard mask indicates that we don’t care about corresponding bit in network address.

OSPF is a classless protocol. With wildcard we can also filter Subnetted networks. In classes implementation usually we use Subnetted networks. For example consider following figure

We have four networks 172.168.1.0/24, 172.168.2.0/24, 172.168.3.0/24 and 172.168.4.0/24 subnetted from single class B network 172.168.0.0/16. Classful configuration does not understand the concept of subnetting. In classful configuration all these networks belong to a single network. Classful configuration works only with in default boundary of mask. Default boundary of this address is 16 bits. So a classful routing protocol will match only first 16 bits (172.168.x.y) of network address. A classful routing protocol such as RIP cannot distinguish between different Subnetted networks.

A classless routing protocol such as OSPF goes beyond the default boundary of mask and work well with Subnetted networks. With wildcard mask we can easily filter Subnetted networks.

With wildcard we are no longer limited with default boundaries. We can match Subnetted networks as well as default networks.

For example we want to exclude serial interfaces in above configuration. We can use a wildcard mask of 0.0.0.255 to match the subnet mask of /24.

Router(config-router)# network 172.168.1.0 0.0.0.255
Router(config-router)# network 172.168.2.0 0.0.0.255

Above commands will ask router to match /24 bits of address instead of default /16 bits. Now router will look for 172.168.1.x and 172.168.2.x network. Our serial interfaces have 172.168.3.0/24 and 172.168.4.0/24 networks which do not fall in these search criteria.

Let’s take one more example, if we use following network command, which interfaces would be selected.

Router(config-router)# network 192.168.0.0 0.0.0.3

In this case valid host IP addresses are 192.168.0.1 and 192.168.0.2. So any interface that has these IP address would be selected. /30 network is usually used for serial link connection which need only two valid host IP addresses; one for each end.

If you are unfamiliar with wildcard mask, I suggest you to check our tutorials on access lists configuration in this category. In those tutorials wildcard masks are explained in detail with examples.

For this tutorial let’s move on third argument. Third argument which network command accept is area number. This parameter say router to put matched interface in specified area. OSPF areas are explained in second part this article.

Now we know the essential commands for configuration. Let’s implement them in our network.

OSPF configuration

Router0

Router1

Router2

Router6

Router5

Router4

Router3

That’s it. Our network is ready to take the advantage of OSPF routing. To verify the setup we will use ping command. ping command is used to test the connectivity between two devices.

We have two routes between source and destination. tracert command is used to know the route which is used to get the destination.

Access the command prompt of PC1 and use ping command to test the connectivity from Server0. After that use tracert command to print the taken path.

Great! We have successfully implemented OSFP routing in our network.

Summary

That’s all for this part. In next part of this article I will explain how OSPF calculate the shortest path for destination.