How to Configure OSPF in a Single Area

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					   Cisco Routing Study Guide: Configuring OSPF in a Single Area
OSPF stands for Open Shortest Path First and it represents a routing protocol
belonging to the group of link-state routing protocols.

OSPF routing protocol is classified among the best dynamic protocols that exist in
networks today. It is also considered to be a sophisticated routing protocol that aims
to maintain loop-free and accurate routing tables.

Today, we’ll go over the necessary commands that are needed to enable the OSPF
routing protocol, define OSPF networks, configure advanced options, and more. We’ll
also look at how and why OSPF is used.

Let’s get started by taking a look at some
Unlike distance vector protocols, link-state routing protocols, and specifically OSPF,
hold the following key points:
     Information about its neighbors (local connectivity) is sent to the entire
        network using multicasting
     The entire routing table is transmitted once every 30 minutes
     Link-state updates (LSAs) are transmitted when there is a change in the state
        of the links
     HELLO messages are used to maintain adjacent neighbors
     Is a classless routing protocol which uses VLSM and both manual and
        automatic summarization
     Uses COST as a metric which CISCO defines as the inverse of the bandwidth
     AREA terminology is created to provide a hierarchical structure and limit the
        multicast LSAs within routers of the same area — the default or otherwise
        core area is area 0 and all other areas connect directly to it

Based on OSPF LSAs, each router constructs a topology table which contains every
connection link within the network. Then, the Dijkstra algorithm runs over the
topology table to find the shortest path to every other router, and hence creates the
routing table.

This algorithm, which is also known as the SPF algorithm, runs on every OSPF
enabled router on the network, and routers within a particular area all have the
same topology tree of the specific area.

Enabling the OSPF Routing Protocol
The following command is needed in order to enable OSPF routing protocol on the
    Router(config)#router ospf process-number

The process-number is nothing more than a number local to the router. It’s only used
to distinguish processes within a router and can be given an arbitrary value. This
value does not have to be the same on every router within the area. However, it is
always good practice to keep this number the same for better administration.

Defining OSPF Networks
Enabling OSPF is not enough to activate it. The OSPF process needs to know the
networks that are going to be advertised (i.e. the interfaces on which OSPF will run)
and the area they reside in. Therefore the following command is needed to make
OSPF operational:
    Router(config-router)#network address wildcard-mask area area-number

The address can be the network address, subnet, or the address of a specific

The network command is used to identify the interfaces on the router that are going
to participate in the OSPF process. Adjacencies will be created with these interfaces
and LSAs will be received and transmitted on these interfaces.

Therefore the wildcard-mask parameter needs to be defined for accurately
identifying the necessary interfaces.

The wildcard-mask consists of 4 groups of 8-bits each. Each 0 bit indicates a “must”
and each 1 bit indicates an “any”. This will become clearer in the next section on
Defining OSPF Networks Examples.

The area-number specifies the area to be associated with the specific address and
consequently the interfaces to be grouped within that area.

By default, area 0 is used; if more than one area is to be created in a network, area
0 is the first one that needs to be defined.

Defining OSPF Networks Examples
The schematic diagram below illustrates how a single command is able to cover all
router interfaces and also how individual interfaces can be specified.

In the diagram above, RouterA has 3 interfaces, one in the subnet, one
in the subnet and the other in the The following command
configures all interfaces to participate in OSPF area 0:
     Router(config-router)#network area 0

The following command will force only interfaces addressed from to
participate in OSPF:
    Router(config-router)#network area 0

The following command specifically forces the serial interface to participate in OSPF
area 0:
    Router(config-router)#network area 0

Configuring Advanced Options
A few advanced options can be useful in administrating OSPF, though they are not
necessary for OSPFs normal operation. These options include:
     Configuring a loopback interface
     Specifying the router ID (RID)
     Manipulating the cost command
     Changing the routers priority

Configuring a Loopback Interface
The source of Link-state Advertisements in a given area is identified by the RID. This
ID has the form of an IP address and can be automatically or manually defined.

Automatic selection of RID
If no manual RID is specified, then the RID is taken from the highest IP address
assigned to a loopback interface. If no loopback interface is defined, then the highest
IP address of an active interface is chosen as the RID.

It is wise to configure a loopback interface on the router because this kind of
interface is a virtual one that can never go down; hence the RID will never have to
change. The following commands define and configure a loopback interface:
     Router(config)#interface loopback interface-number
     Router(config-if)#ip address ip-address subnet-mask

Manual Configuration of RID
This is the preferred method of defining the RID. Any unique IP address can be
defined as RID and it never changes even if the interface that the router is using for
the RID goes down.

Even when a manual configuration change of the RID is performed, this does not take
effect until the next router reload or OSPF routing restart.

The following commands show how to manually configure the RID. The clear ip ospf
process command is used to activate the RID on a router that is already running
    Router(config)#router ospf process-number
    Router(config-router)#router-id ip-address
    Router#clear ip ospf process

Configuring the Cost of an Interface
As already mentioned, the OSPF process assigns cost values to interfaces based on
the inverse of the bandwidth parameter assigned to the interface with
the bandwidth command.

For calculating the SPF to a given destination, the router takes into consideration the
costs of the links along various paths. The path with the lower cost is selected as the
shortest path.

To force the router to pick up a certain path, manual setting of the cost on a link
along the path can be achieved using the following command:
     Router(config-if)#ip ospf cost cost

Setting Priority for the Selection of the Designated Router
In multi-access networks the router with the highest priority value is chosen as the
DR which acts as the central point of LSAs exchange.

The priority command is assigned on an interface. Default priority for an OSPF
interface is 1. The range is from 0 to 255. 0 means that the interface does not involve
in the DR election.
The following command configures a priority value on an interface.
      Router(config)#interface interface-number
      Router(config-if)#ip ospf priority priority-value

Why Use OSPF?
As I mentioned at the beginning, OSPF routing protocol is classified among the best
dynamic protocols existing in networks today. It has all the necessary features of an
interior gateway routing protocol:
     Scalability
     Adaptability to topology changes
     Topology awareness within its operation area

OSPF uses multicasting so that each router informs all other routers in the area of its
neighbors and costs. This information allows each router to build a complete
topological tree consisting of routers and omnidirectional links connecting them
together. Each of these links carries a cost value.

SPF algorithm runs over this topology tree and computes the shortest path from
every router to every other router in the area. The results from SPF algorithm
construct the router’s routing table.

More Cisco Routing Study Tips:
How to Troubleshoot OSPF?
OSPF, How to Configure OSPF in the Cisco IOS?


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