OSPF Network Types

Introduction

If you have ever heard of an OSPF network routing protocol, you might be wondering what they actually entail. There are in fact different network types that are supported by OSPF. The types all behave a little different between configurations. Here’s how it works.

What Is OSPF?

It is important to understand what OSPF is before we go any further. The acronym stands for “open shortest path first.” OSPF is a routing protocol for IP networks that uses a link sat routing algorithm. It is an IGP and is normally found in enterprise networks rather than service provider networks.

To put it more simply, the OSPF protocol is looking for the shortest route to a destination. It uses the network’s algorithm to calculate out which path makes the most sense. Of course, this decision happens in a split second with all of the work happening on an unseen level.

Routing Protocol

A routing protocol operates through the Request for Comments (RFC) process and involves open written commentary in an effort to make this type of technology standard. Routed protocols forward data through routers. Routers must be able to interpret the data in order for it to use it correctly. There are also dynamic routers that support a routed protocol through routing tables. OSPF is one such protocol.

Hellos

OSPF also use what is known as the “hello procedure.” Hello packets are sent by OSPF routers, which will change the cost of links to neighbouring routers. Hellos are used through multicast when routers are able to detect one another and the wired routers send hello packets out to each other. Then, they can have a two-way relationship, as long as all of the parties are amicable.

Types Of OSPF Networks

Now, let’s take a look at the different types of OSPF networks that are out there.

• Point-to-point: A point-to-point network is a single circuit that connects multiple OSPF routers. The purpose is to create a single-neighbour relationship. It is often seen in PPP or HDLC in this case. This is the simplest type of network.
• Point-to-multipoint: With a point-to-multipoint setup, OSPF is allowed to operate like the routers are connected with point-to-point links and not an NBMA network. Every link in this configuration is treated as if it were a point-to-point link.
• Broadcast: With this OSPF configuration, two or more OSPF routers are connected using an Ethernet or FDDI connection. With the use of OSPF Hellos, neighbour relationships are developed. A designated router and back up router are made with adjacencies to them. A broadcast interface has Layer 2 capabilities, which is standard.
• Non-broadcast multi-access: An NBMA network does not allow broadcasts automatically. Frame relay and ATM are both examples of an NBMA. They also have the potential for multiple adjacencies. They do not all, however, send broadcasts and cannot be sure that the right relationships will in fact form.

Important Note

If two OSPF routers would like to establish adjacency across a common link, then the routers have to be on the same OSPF network type. If there are different type sat plays, there will be a lot of frustrations, because the interoperability is inevitable. It is the most common reason for OSPF failure, so make sure to see how your router is configured before trying to set up anything else.