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:

• 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 interface.

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 192.168.7.0 subnet, one in the 192.168.8.0 subnet and the other in the 172.16.1.0. The following command configures all interfaces to participate in OSPF area 0:

• Router(config-router)#network 0.0.0.0 255.255.255.255 area 0

The following command will force only interfaces addressed from 192.168.0.0 to participate in OSPF:

• Router(config-router)#network 192.168.0.0 0.0.255.255 area 0

The following command specifically forces the serial interface to participate in OSPF area 0:

• Router(config-router)#network 172.16.1.1 0.0.0.0 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 OSPF:

• 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.

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1. Cisco BSCI Exam Study Guide on Dynamic Routing Protocols
2. IP Addressing and Routing Part 3: Default & Static Routing Basics
3. How to Configure RIP (Routing Information Protocol) Part 2
4. Cisco CCENT Study Guide: WAN Protocol Basics
5. Cisco IS-IS Configuration Primer

The BSCI exam is all about advanced IP addressing and routing and it tests your knowledge and skills on implementing scalability for Cisco Integrated Services Routers (ISR) connected to LANs and WANs.

Some of the topics that the BSCI exam covers include:

• Advanced IP addressing
• Routing principles
• Multicast routing
• IPv6
• Manipulating routing updates
• Configuring basic BGP
• Configuring EIGRP, OSPF, and IS-IS

In the past, I covered some of the above mentioned topics, but today I will concentrate on Dynamic Routing Protocols. More specifically, I’ll introduce you to the two major classes of Dynamic Routing Protocols: Distance Vector and Link State.

Before we get started, if you need to refresh your memory on static and default routing concepts, then take a look at IP Addressing and Routing Part 3: Default and Static Routing Basics.

Now let’s begin with a description of the operational principles of the two routing classes and afterwards, I’ll get into the details on their actual operation and design.

Why Two Dynamic Routing Protocol Classes?

There are different routing classes available for providing a more spherical solution packet. Different networks have special individual needs and different routing protocols have been designed to meet the individual needs of these networks.

There is no straightforward answer on the right routing protocol to use. A variety of parameters need to be investigated before deciding on that. Your investigations should include bandwidth prerequisite, reliability, convergence speed, network architecture and much more.

I won’t concentrate on the details of the best routing decision process, but I will try to illustrate the details behind the operation of the different routing classes so that you can make the appropriate decisions yourself.

Distance Vector Routing Protocols

Distance Vector routing protocols base their decisions on the best path to a given destination based on the distance. Distance is usually measured in hops, though the distance metric could be delay, packets lost, or something similar.

If the distance metric is hop, then each time a packet goes through a router, a hop is considered to have traversed. The route with the least number of hops to a given network is concluded to be the best route towards that network.

The vector shows the direction to that specific network. Distance vector protocols send their entire routing table to directly connected neighbors. Examples of distance vector protocols include RIP – Routing Information Protocol and IGRP – Interior Gateway Routing Protocol.

If you’re interested in finding out more information on RIP, check out: How to Configure Routing Information Protocol Part 1 on RIPv1 and Part 2 on RIPv2.

Link State Routing Protocols

Link state protocols are also called shortest-path-first protocols. Link state routing protocols have a complete picture of the network topology. Hence they know more about the whole network than any distance vector protocol.

Three separate tables are created on each link state routing enabled router. One table is used to hold details about directly connected neighbors, one is used to hold the topology of the entire internetwork and the last one is used to hold the actual routing table.

Link state protocols send information about directly connected links to all the routers in the network. Examples of Link state routing protocols include OSPF – Open Shortest Path First and IS-IS – Intermediate System to Intermediate System.

There are also routing protocols that are considered to be hybrid in the sense that they use aspects of both distance vector and link state protocols. EIGRP – Enhanced Interior Gateway Routing Protocol is one of those hybrid routing protocols.

Distance Vector Routing Protocols – Operation

To illustrate the routing updating process for Distance Vector routing protocols I will use the following subnet diagram:

Let’s say that all routers have been set in service at the same time and all run a distance vector routine protocol. Each router sends its distance vector to its neighbor. Also each router receives distance vectors from each neighbor as well.

Combining the information learned from neighbors with each router’s own information, the best estimate route to a given destination is inserted into the routing table.

Let’s take a look at how Router H builds up its routing table after receiving Hop Distance Vectors from its neighbors (Routers A, B, F, G and I). For each neighbor router, a separate vector table is shown:

Upon receiving the routing updates from all neighbors, RouterH performs its calculation for estimating the best route to any given destination. The result of this process is the following:

To give you an idea of how this calculation process is performed consider the following:

RouterA and RouterG informed RouterH that RouterD is 1 hop away. RouterH knows that both routers (A and G) are neighbor routers; hence it adds 1 to the hop metric and concludes that it can reach RouterD via both RouterA and RouterG with an overall distance of 2 hops.

Link State Routing Protocols – Operation

As already mentioned, Link State routing protocols hold 3 distinctive tables: a neighbor table, a topology table, and an actual routing table. Link state routing operation follows four simple steps; each link state enabled router must perform the following:

1. Discover its neighbors and build its neighbor table.
2. Measure the cost (delay, bandwidth, etc) to each of its neighbors.
3. Construct and send a routing update telling all it has learned to all routers in the network.
4. Apply the Dijkstra algorithm to construct the shortest path to all possible destinations.

Below you’ll find more details on the four step process.

Step 1: Neighbor Discovery

Each Link State enabled router periodically sends a HELLO message on each of its links. Neighbor routers respond to these HELLO messages identifying themselves. Within the replies, network addresses of the routers are attached and are used by the HELLO initiator to build up its neighbor table.

Step 2: Measuring Link Cost

A set of tests is performed on each router to measure the cost to each of its neighbors. The cost could be a measure of the end-to-end delay, throughput, or a combination of these metrics. How these tests are performed is out of the scope of this article. The important thing to know is that each link state enabled router has to somehow possess an estimate of the cost for each of its links.

Step3: Building and Distributing Link State Packets

Each router builds up a packet containing its neighbors and the corresponding link costs to these neighbors. At the beginning of the packet, each router adds its identity along with a sequence number and an age parameter, the latter being used to ensure no packet will wander around for an indefinite period of time. After the construction process, the packet is flooded in the network.

Based on the same subnet diagram presented above the equivalent link state packets for each of the routers are presented below. Random link cost values have been assumed. Also for simplicity, sequence and age values are omitted.

Step 4: Evaluating Shortest Paths

Using all the details from its link state table, a router is able to compute, using the Dijkstra algorithm, the shortest path to any given destination.

For example, the best path for RouterA to reach RouterI would be the following:

Things to Keep in Mind

• Distance vector protocols send their entire routing table to directly connected neighbors.
• Link state protocols send information about directly connected links to all the routers in the network.
• Distance vector protocols have slow convergence and suffer from the count-to-infinity problem details of which you can find here.
• Link state routing protocols are widely used in large networks due to their fast convergence and high reliability.

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Related posts:

1. Cisco Routing Study Guide: Configuring OSPF in a Single Area
2. How To Configure RIP (Routing Information Protocol) Part 1
3. Master EIGRP For Your Next Cisco Exam
4. How to Configure Interior Gateway Routing Protocol (IGRP)
5. IP Addressing and Routing Part 3: Default & Static Routing Basics