Understanding OSPF: How It Works and Its Characteristics

We all know that there are several types of dynamic routing protocols, one of which is OSPF. This time we will discuss about OSPF starting with understanding OSPF and how it works and its properties.

OSPF itself is a well-known routing protocol because this routing protocol is best suited for use in medium-sized enterprise LANs. For example in offices that use more than 50 computers and other devices, or in companies that have many branches with many computer clients, multinational companies with many branches abroad, and many more. Why is it said that it is the most suitable? Because OSPF has a high level of scalability, reliability, and compatibility.

Read also: Understand routing tables and routing protocols

Understanding OSPF

OSPF (Open Shortest Path First) is an IGRP (Internal Gateway Routing Protocol) type routing protocol that can only work within an organization’s internal network. Intranet means a network that you may still use, manage and modify. Or in other words, you still have administrative rights to the network. If you no longer have the rights to use and manage it, the network can be classified as an external network.

In addition, OSPF is also an open standard routing protocol. The point is that this routing protocol is not made by any vendor. Thus, anyone can use it, any device it can be compatible with, and wherever this routing protocol can be implemented.

OSPF is a routing protocol that uses the concept of a routing hierarchy, which means that OSPF divides the network into several levels. These levels are achieved using the zone aggregation system. Using the concept of a routing hierarchy, the information dissemination system becomes more organized and fragmented, and it does not spread here and there randomly.

The effect of routing distribution uniformity is that the network uses bandwidth more efficiently, achieves convergence faster, and is more accurate in determining the best paths to the site. OSPF is a routing protocol that always tries to work this way.

The technology used by this routing protocol is a link-state technology designed to work very efficiently in the process of sending route information updates. This makes the OSPF routing protocol very suitable for continuous development in large-scale networks. OSPF users are typically intermediate to senior network administrators. Networks with more than ten routers, with many remote sites to be accessed from the center, and more than five hundred network users, it may be possible to use this routing protocol.

How does OSPF work

During the implementation of the routing information exchange, OSPF will establish a connection with other routers. Other routers that are directly connected to or in the same network with an OSPF router are called neighbor routers or neighbor routers.

OSPF routers have a mechanism for finding neighboring routers and opening a connection. This mechanism is known as the welcome protocol.

When establishing a connection to its neighbors, the OSPF router will periodically send a small packet to the network or to a device directly connected to it. The small package is called the hello package. Under standard conditions, Hello packets are sent periodically once every 10 seconds (in multi-access broadcast media) and every 30 seconds in Point-to-Point media.

Hello packets contain information about the ingenuity on the sending router. Welcome packets are generally sent using a multicast address to all OSPF (IP multicast routers. All OSPF routers will listen for this greeting protocol and send greeting packets periodically. Hello protocol work and the creation of adjacent routers are of several types, depending on the type of media on which the OSPF router is operating.

However, the unique way in which Hello protocol works on OSPF varies with the type of media. There are several types of arguments that can pass OSPF information, and each has its own characteristics, so OSPF also works according to its own characteristics. The media are as follows:

multi-access broadcast

This type of media is media that is widely available in LANs or LANs such as Ethernet, FDDI, and token ring. In media conditions like these, OSPF will send multicast traffic in search of nearby routers. However, there is something unique about this media process, which is that two routers that are running will be identified as Mapped Routers (DR) and Mapped Routers for Backup (BDR).

point to point

Point-to-Point technology is used in circumstances where there is only one other router connected directly to the router. An example of this technique is serial linking. In this “point to point” situation, the OSPF router does not need to create and backup a specific router because there is only one router that needs to be used as a neighbor. In this adjacency lookup, the OSPF router will also send hello packets and other messages using the multicast address called AllSPFRouters

point to point

This type of media is media with a single interface that connects it to several purposes. The networks below are a series of point-to-point networks that are directly connected to the main device. OSPF routing protocol messages will be replicated over a point-to-point network. In this type of network, OSPF traffic is also transmitted using multicast IP addresses. But what distinguishes it from multi-access broadcast media is that there is no specific router selection and backup as it does not forward broadcasts.

Non-Broadcast Multiple Access (NBMA)

These non-broadcast multiple access media are physically a regular serial line often found in point-to-point media. But in fact, this medium can provide connections to many destinations, and not just to one point. An example of this medium is X.25 and frame relay which are well known for providing solutions for dispersed offices. In the use of these media, two types of usage are known, namely partial mesh and fully meshed. OSPF sees this type of media as a multi-access broadcast medium.

But in fact, this notification cannot send the broadcast to the points in it. Therefore, to apply OSPF in these media, manual configuration of DR and BDR is required. Once DR and BDR are selected, the DR router will create an LSA for the entire network. In this type of media, DR and BDR are routers that have direct connections to all of their neighboring routers. All traffic sent from neighboring routers will be copied by DR and BDR to each router and sent using a unicast address or like an OSPF process on point-to-point media.

OSPF properties

OSPF routing protocol has the following characteristics:

  • It is a link-state routing protocol, so that each router has a file
    Network topology overview.
  • Using Hello Packet to find out where nearby routers are
    (Neighboring routers).
  • Routing updates are only sent when network changes occur and
    Sent by multicast.
  • Can work with hierarchical concepts because they can be divided by concept
  • Use cost as a measure of lowest cost
    The best metrics.
  • It has no limit on the number of hops other than RIP can only
    Stretch 15 jumps.
  • It is a classless routing protocol.
  • By default, the administrative distance value is 110.
  • It has an authentication feature when sending routing updates.

Advantages and disadvantages of OSPF

Advantages of OSPF:

  • OSPF uses the network partition based on the concept of regions.
  • The concept of a hierarchical network, which makes the process of updating information better managed.
  • The presence of convergence, whereby the router will receive information from other routers that act as neighbors, so that eventually all the information in the network can be known by all the routers in the network.
  • The system updates routing information fairly regularly.
  • OSPF saves on network bandwidth usage.
  • OSPF uses cost as a metric.

OSPF disadvantages:

  • It requires a large database.
  • It consumes a lot of resources.
  • It requires planning in its design and implementation in a network.


So Open Shortest Path First is an automatic routing protocol (dynamic routing) capable of maintaining, managing and distributing routing information between networks after any dynamic changes in the network. There are 5 types of packages used by OSPF, which are:

  1. Hi packets to find and establish a connection between nearby OSPF routers.
  2. Database Description (DBD) to verify database synchronization between routers.
  3. Link-State Request (LSR) specifications for link state records between one router and another.
  4. Link State Update (LSU) sends a request to identify link state records.
  5. A link state acknowledgment (LSAck) accepts link state packets.

OSPF is also a routing protocol that has very impressive features and capabilities, especially for the internal network of an organization or company. Compared with RIP and IGRP, which are both IGP (Internal Gateway Protocol) routing protocols, OSPF is more powerful, scalable, flexible and feature-rich.

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