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Articles and examples provided in this section, cover the CCENT, CCNA, CCDA, CCNP & CCVP certification levels.

This article covers basic OSPF concepts and operation. We explain how OSPF works, how OSPF tables are built on an OSPF-enabled router and their purpose (Neighbour Table, Topology Table, Routing Table), OSPF areas and their importance. Next we cover OSPF Link State Packet types used to exchange data between OSPF routers: Link State Advertisement (LSA), Link State Database (LSDB), Link State Request (LSR), Link State Update (LSU) and Link State Acknowledgment (LSAcK). Finally, we take a look at the OSPF roles: Area Boarder Router (ABR), Autonomous System Boundary Router (ASBR), Designated Router (DR), Backup DR and more.

 What is OSPF and How Does it Work?

OSPF is a Link State protocol that’s considered may be the most famous protocol among the Interior Gateway Protocol (IGP) family, developed in the mid 1980’s by the OSPF working group of the IETF.

When configured, OSPF will listen to neighbors and gather all link state data available to build a topology map of all available paths in its network and then save the information in its topology  database, also known as its Link-State Database (LSDB). Using the information from its topology database. From the information gathered, it will calculate the best shortest path to each reachable subnet/network using an algorithm called Shortest Path First (SFP) that was developed by the computer scientist Edsger W. Dijkstra in 1956. OSPF will then construct three tables to store the following information:

• Neighbor Table: Contains all discovered OSPF neighbors  with whom routing information will be interchanged
• Topology Table: Contains the entire road map of the network with all available OSPF routers and calculated best and alternative paths.
• Routing Table: Contain the current working best paths that will be used to forward data traffic between neighbors.

Understanding OSPF Areas

OSPF offers a very distinguishable feature named: Routing Areas. It means dividing routers inside a single autonomous system running OSPF, into areas where each area consists of a group of connected routers.

The idea of dividing the OSPF network into areas is to simplify administration and optimize available resources. Resource optimization is especially important for large enterprise networks with a plethora of network and links.  Having many routers exchange the link state database could flood the network and reduce its efficiency – this was the need that led to the creation of concept Areas.

Areas are a logical collection of routers that carry the same Area ID or number inside of an OSPF network, the OSPF network itself can contain multiple areas, the first and main Area is called the backbone area “Area 0”, all other areas must connect to Area 0 as shown in the diagram below:

Figure 1. OSPF Areas, Area 0 (Backbone Area), ABR and ASBR OSPF routers

All routers within the same Area have the same topology table -Link State Database- but different routing table as OSPF calculates different best paths for each router depending on its location within the network topology while they will all share the same Link State topology.

The goal of having an Area is to localize the network as follow:

One of the most difficult things for people who are starting out in a networking career is getting their hands on the equipment. Whether you are studying for Cisco certification or just wanting to test certain network behaviors in a lab, no one would argue that practicing is the best way to learn.

I have seen people spend hundreds or thousands of dollars (myself included) buying used networking equipment in order to build a home Cisco lab to gain practical experiences and study for certification exams. Until a few years ago it was the only option available, or you had to rent lab hours through one of the training companies.

Other Simulation Tools

GNS3 is a well-known free network simulation platform that has been around for many years. Cisco IOS on UNIX (IOU) is another option for running Cisco routers in a virtual environment. It is a fully working version of IOS that runs as a user mode UNIX (Solaris) process. IOU was built as a native Solaris image and runs just like any other program. One key advantage that Cisco IOU has is that it does not require nearly as much resources as GNS3 and VIRL would require. However, the legality of the source of Cisco images for GNS3 is questionable.

Figure 1. Cisco VIRL Network Topology

If you are not an authorized Cisco employee or trusted partner, usage of Cisco IOU is potentially a legal gray area. Because of lack of publicity and availability to average certification students and network engineers, online resources are limited and setting up a network takes much more effort. Also, due to missing features and delays in supporting the recent Cisco image releases, Cisco is not recommending them to engineers and students.

Read our review on "The VIRL Book" – A Guide to Cisco’s Virtual Internet Routing Lab (Cisco Lab)

Here Comes Cisco VIRL

Cisco Virtual Internet Routing Lab (VIRL) is a software tool Cisco developed to build and run network simulations without the need for physical hardware.

Routing is one of the most important features in a network that needs to connect with other networks. In this page we try to explain the difference between Routed and Routing protocols and explain different methods used to achieve the routing of protocols.The fact is that if routing of protocols was not possible, then we wouldn't be able to comminucate using computers because there would be no way of getting the data across to the other end !

Definition

Routing is used for taking a packet (data) from one device and sending it through the network to another device on a different network. If your network has no routers then you are not routing. Routers route traffic to all the networks in your internetwork. To be able to route packets, a router must know the following :

• Destination address
• Neighbor routers from which it can lean about remote networks
• Possible routes to all remote networks
• The best route to each remote network
• How to maintain and verify routing information

Before we go on, I would like to define 3 networking terms :

Convergence: The process required for all routers in an internetwork to update their routing tables and create a consistent view of the network, using the best possible paths. No user data is passed during convergence.

Default Route: A "standard" route entry in a routing table which is used as a first option. Any packets sent by a device will be sent first to the default route. If that fails, it will try alternative routes.

Static Route: A permanent route entered manually into a routing table. This route will remain in the table, even if the link goes down. It can only be erased manually.

Dynamic Route: A route entry which is dynamically (automatically) updated as changes to the network occur. Dynamic routes are basically the opposite to static routes.

We start off with the explanation of the IP routing process and move onto routed protocols, then tackle the routing protocols and finally the routing tables. There is plenty to read about, so grab that tea or coffee and let's start!

Supernetting, also known as Classless InterDomain Routing (CIDR), is another awesome subject. It exists thanks to the wide adoption of the Internet, which lead to the exhaustion of the available IP Addresses. More specifically, supernetting was invented in 1993 with the purpose of extending the 32 bit IP address lifetime until the adoption of IPv6 was complete.

Putting it as simply as possible, supernets are used to combine multiple Class C networks into groups, which the router, in turn, treats as one big network. It might not seem like a smart thing to do, but if you look at the picture on a larger scale you will notice some of the really awesome advantages this offers.

The creation of Supernets is also known as Address Aggregation.

The sections below analyse Supernetting and CIDR, both which are extremely important to understand since they are used by all ISP's and large networks.