Networking Basics - OSPF Theory

ccie r/s ccna r/s ccnp r/s Mar 18, 2021


In my latest training video, I cover the theory of Open Shortest Path First (OSPF) routing protocol.


OSPF is the most popular interior gateway protocol used today. In this video, you’ll understand:

  • How link state databases are constructed with information from link state advertisements (LSAs)
  • The difference between an OSPF Neighbor and an OSPF Adjacency
  • OSPF areas
  • How OSPF performs its cost calculations


Enjoy the training!

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OSPF Advanced Concepts - Part 4

ccie r/s ccna r/s ccnp r/s Sep 24, 2019

In the previous part of our OSPF series, we examined options for manually filtering routes. As we wrap up our look at advanced OSPF topics, we'll discuss default routes, and compare OSPFv2 with OSPFv3.

Default Routes

We have seen where OSPF can automatically generate a default route when needed. This occurs with some of our special area types. For example, of you configure a totally stubby area, of course a default route is required and OSPF generates this route automatically from the ABR.

In order to increase flexibility with your designs, default routes injected into a normal area can be originated by any OSPF router. To generate a default route, you use the default-information originate command.

This command presents two options:

  • You can advertise into the OSPF domain, provided the advertising router already has a default route.
  • You can advertise regardless of whether the advertising router already has a default route. This second method is accomplished...
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OSPF Advanced Concepts - Part 3

ccie r/s ccna r/s ccnp r/s Sep 10, 2019

In our previous blog post, we examined how OSPF can automatically filter routes through the use of special areas and LSA Types. But what about your options for manually filtering routes in OSPF? In this post, we will examine techniques that you can use at various points in the topology.

Filtering at the ASBR

One simple and effective method of filtering at the ASBR is the use of a distribute list. Here, we define the rules for route identification with an access list, and then reference this access list in the distribute list. 

Figure 1 - OSPF Topology

In this example, our Area 1 is configured as a normal, non-backbone area. You can clearly see this when you examine the routing table on ORL.

Note the two prefixes (E2) of and Let’s filter at the ASBR of ATL.

Note how simple this configuration is. Let’s see if it worked by examining the route table of ORL once again:

The configuration worked perfectly and...

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OSPF Advanced Concepts - Part 2

ccie r/s ccna r/s ccnp r/s Aug 27, 2019

Last time, we began our look at advanced OSPF topics with the configuration of backbone and non-backbone areas. In this blog post, we'll look at the creation of more specific area types.

Stubby Area

Figure 1 - OSPF Topology

It is time to make our Area 1 from Figure 1 a stubby area. This is a simple configuration change. On each device in the area, we need to set the Area 1 as stub. Here is the configuration in our network:

This will cause a reset of the adjacency (as you might guess). After this change, it is time to check the OSPF route table and the OSPF database on ORL:

Just as we would hope, the routing table is smaller now! There is no longer the detail of the external prefixes from the ASBR. Instead we have a default route automatically generated by the ABR. This default route is needed, of course, because the routers in Area 1 still need to be able to access the remote prefixes (if needed).

Now it is time to examine the OSPF database. It is exactly what we would expect to...

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OSPF Advanced Concepts - Part 1

ccie r/s ccna r/s ccnp r/s Aug 13, 2019

The time has arrived to tackle some of the more advanced (and interesting) features of the Open Shortest Path First routing protocol. We begin by examining the configuration and verification of the different OSPF areas. This is an exercise that is not only fun, but it can really cement the knowledge down of how these areas function and why they exist.


Areas are a fundamental concept of OSPF. It is what makes the routing protocol, hierarchical, as we like to say.

There is a core backbone area (Area 0) that connects to normal, non-backbone areas. The backbone might also connect to special area types we will examine in detail in this chapter. This hierarchical nature of the design helps ensure the protocol is very scalable. We can easily reduce or eliminate unnecessary routing traffic flows and communications between areas if needed. Database sizes are also contained using this approach.

The Backbone and the Non-Backbone Areas

To review a bit from our previous blog...

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OSPF Basic Concepts - Part 3

ccie r/s ccna r/s ccnp r/s Jul 30, 2019

Before we move on to more advanced topics, we'll wrap up this OSPF Basics series in Part 3. Here we'll examine LSA types, area types, and virtual links.


Link State Advertisements (LSA) are the lifeblood of an OSPF network. The flooding of these updates (and the requests for this information) allow the OSPF network to create a map of the network. This occurs with a little help from Dijkstra’s Shortest Path First Algorithm. 

Not all OSPF LSAs are created equal. Here is a look at each:

The Router (Type 1) LSA - We begin with what many call the “fundamental” or “building block” Link State Advertisement. The Type 1 LSA (also known as the Router LSA) is flooded within an area. It describes the interfaces of the local router that are participating in OSPF and the neighbors the local OSPF speaker has established.

The Network (Type 2) LSA - Remember how OSPF functions on an Ethernet (broadcast) segment. It elects a Designated Router...

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OSPF Basic Concepts - Part 2

ccie r/s ccna r/s ccnp r/s Jul 23, 2019

In the previous blog post, we looked at a few fundamental OSPF concepts, including neighbor and adjacency formation. As we continue through the basics of OSPF, this post will examine router roles, timers, and metric calculation.

Designated Router (DR) and Backup Designated Router (BDR)

A designated router (DR) is the router interface that wins an election among all routers on a multiaccess network segment such as Ethernet. A backup designated router (BDR) is the router that becomes the designated router if the current designated router has a failure on the network. The BDR is the OSPF router with the second highest priority at the time of the last election. OSPF uses the DR and BDR concept to assist with efficiencies in the operations of OSPF.

Keep in mind that a given OSPF speaker in your network can have some interfaces that are designated and others that are backup designated, and others that are non-designated. If no router is a DR or a BDR on a given...

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OSPF Basic Concepts - Part 1

ccie r/s ccna r/s ccnp r/s Jul 09, 2019

The OpenShortest Path First (OSPF) dynamic routing protocol is one of the most beloved inventions in all of networking, widely adopted as the Interior Gateway Protocol (IGP) of choice for many networks. In this blog series, you'll be introduced first to the basic concepts of OSPF and learn about its various message types and neighbor formation.

An Overview of OSPF

Where does the interesting name come from when it comes to OSPF? It is from the fact that it uses Dijkstra's algorithm, also known as the shortest path first (SPF) algorithm. OSPF was developed so that the shortest path through a network was calculated based on the cost of the route. This cost value is derived from bandwidth values in the path. Therefore, OSPF undertakes route cost calculation on the basis of link-cost parameters, which you can control by manipulating the cost calculation formula.

As a link state routing protocol, OSPF maintains a link state database. This is a form of a network...

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Fundamentals of Border Gateway Protocol (BGP) - Part 6

ccie r/s ccna r/s ccnp r/s Jun 04, 2019

This post is the 6th and final in a series of Border Gateway Protocol (BGP) posts. If you missed any of the first five parts, here are the links:

In this post, we're going to take a look at how we can work with BGP in IPv6.

BGP in IPv6

You will recall from this chapter that BGP was constructed to support many different protocols and NLRI right out from its creation. As a result, we have robust support for such technologies as IPV6, MPLS VPNs, and more.

You will also relish in the fact that once you master the basics of BGP that we have covered in this , working with BGP in IPv6 is much more similar than it is different!

BGP is so remarkably flexible, as discussed earlier in this chapter, you can use IPv4 as the “carrier” protocol for IPv6...

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Fundamentals of Border Gateway Protocol (BGP) - Part 5

ccie r/s ccna r/s ccnp r/s May 21, 2019

This post is the 5th in a series of Border Gateway Protocol (BGP) posts. If you missed any of the first four, here are the links:

In this post, we're going to take a look at BGP scalability mechanisms and related concepts.

BGP Scalability Mechanisms

Just as IP address depletion has been a concern with the Internet, so has the depletion of available autonomous system numbers. To help solve this, the engineers turned to a familiar solution. They marked an AS number range as private-use only. This permits you to experiment with AS construction and policy in a lab (for example) and use AS numbers that are guaranteed not to conflict with Internet-based systems.

Remember, the AS number is a 16-bit number permitting up to 65,536 AS numbers. The private space is marked as 64512-65535.


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