What is OSPF? | Exploring Open Shortest Path First Protocol

The Open Shortest Path First (OSPF) protocol is critical for network routing. In its simplest form, OSPF provides efficient communication between routers. We also know it as a Link State routing protocol. In addition, it uses the Dijkstra algorithm and offers many features and capabilities. In addition, it has VLSM and CIDR support. Thus, it plays a vital role in modern networks.

In this article, I will explain in detail what exactly the OSPF protocol is. I will also compare it to other routing protocols by discussing its functionality and features. I will also explain how to calculate metrics in network topologies.

What is the OSPF Protocol Used for Routing Between Routers in the Network?

OSPF (Open Shortest Path First) is a Link State routing protocol defined in RFC 2328.

Link State routing protocols know the entire network map. In addition, Triggered Updates are sent when there is any change on the network, providing fast network integration.

In a network using the OSPF protocol, when a change occurs, for example, when a new Router is added to the network environment or a Router fails, Routers send link status notifications to each other.

In any change in the network, the Router affected by the shift sends updated LSA packets to the network environment. Other Routers update the network topology with the LSA packets they receive. It also renews their SPF trees and updates their routing tables.

This protocol uses the Dijkstra algorithm and sends updates every 30 minutes. It also supports IPv4 and IPv6, such as EIGRP. It uses a value called Area and divides large networks into areas, reducing the size of the routing table and the complexity of the network.

Also, the OSPF protocol requires high hardware requirements such as RAM and CPU. Expert personnel are required to configure or edit this routing protocol used in large networks.

Features of the OSPF Protocol

If we list the OSPF features,

1. Uses Area and Autonomous System.
2. Minimizes routing updates.
3. Supports CIDR and VLSM.
4. Provides reliability.
5. Unlimited Hop Count feature.
6. It can also be used on non-Cisco devices.
7. The AD (Administrative Distance) value is 110.
8. Provides route authentication.
9. It provides fast integration.
10. Sends updates only when the network changes.
11. It does not send updates to the entire routing table.
12. It uses the SPF algorithm to calculate the lowest cost to a target.
13. Sends Hello packets to neighboring Routers every 10 seconds.
14. In non-broadcast networks, the Hello packet sending time is 30 seconds.

Routing Tables of the OSPF Protocol

1. Neighbor Table

This table stores the information on the neighboring devices that the Router owns. Each Router has its neighbor table. Each Router has an IP address and interface information for neighboring devices.

2. Link State Table

This table holds connection status information for neighboring devices. The routing table on the other Routers contains the link status information, and each Router has a Link State Table.

3. Routing Table

This table holds the metric of each record, followed by the Link State table. As a result, it keeps the route information for the shortest route.

OSPF and RIP Comparison

When you look at the image above, you can see that Router R1 is in the Area0 area, and it is a Backbone Router. And so you can communicate with routers that are connected to different places.

An Example of the OSPF Topology

When OSPF network topology is designed, the Area must be determined. The OSPF routing protocol must have an Area 0, and all other OSPF Areas must be connected to this Area. As an example of OSPF network topology, you can examine the following image.

When we look at the above image, Router R1 is in the Area 0 field and is a Backbone Router. Routers connected to different OSPF areas can communicate with each other.

Metric Calculation

Metric calculation is done according to bandwidth. In short, the bandwidth of each connection is taken into account when calculating this value.

Cisco Router sets its metric to 100 Mbps by default and evaluates accordingly.

Terminology / Terms

• Link: Link is the interface associated with a network.
• Router ID: An IP address used to identify the Router. In the OSPF network, each Router must have RID. By default, the most prominent IP address is selected as the RID. Loopback interfaces are usually defined to determine the router ID.
• Neighbor: A neighbor is a point-to-point connection of one or two Cisco Routers.
• Adjacency: Route information updates are shared directly between the two Routers. Unlike EIGRP, route information is transmitted directly between neighboring Routers.
• Hello Protocol: Uses Hello packets to establish and maintain a neighbor relationship between routers. Hello, packages and LSA packages build and maintain the logical database.
• Neighborship DB: A list of all Routers. Router ID and State information are stored and maintained in the neighborhood database.
• Topological DB: A database created by LSA packets received from an Area. It is used as a record for the Dijkstra algorithm used to find the shortest path.
• Link State Advertisement (LSA): A data package containing link status and routing information.
• Designated Router (DR): A DR Router collects routing information in itself and distributes it to other Routers.
• Backup Designated Router: When the DR Router is not active, the BDR Router is enabled. It receives all routing information from other neighboring devices but does not send LSA packets.
• Area: Area is a grouping of adjacent networks and routers. All routers in the same domain share a standard Area ID. The Area ID is associated with a specific interface on the Router. Routers within the same Area have the same topology table.
• Broadcast (multi-access): A multi-access network consisting of a group of Routers. DR and BDR routers must be selected for the broadcast network.
• Non-Broadcast (multi-access): Non-Broadcast networks. For example, Frame-Relay and ATM are non-broadcast networks.
• Point-to-Point: A network type that provides a direct connection between two Routers that offer a single communication path.
• Point-to-Multipoint: The type of communication on a router with the connection of multiple Router groups.

Configuration Videos

Video 1

Video 2

Conclusion

In conclusion, the Open Shortest Path First (OSPF) protocol is a fundamental component of modern networks. We have examined the OSPF using the Link State routing protocol. In this way, I can say that it provides efficient communication between routers.

In short, the OSPF protocol is a valuable tool for network administrators. When we compare it with other protocols, we can understand its importance. It also optimizes network performance as it creates stable and scalable networks. In summary, it plays a critical role in large networks by providing reliable communication between devices.