How OSPF Protocol works and OSPF basic Concepts

OSPF (Open Shortest Path First) is used to distribute the information of IP routing throughout a single Autonomous System (AS) in an IP network. In order for the OSPF protocol to perform its required function, it collects various data about the network and its devices and then uses this data to calculate the best paths for each subnet.

OSPF collects data about subnets and available paths and collects data about the router's surrounding devices. Then it selects the best paths for each subnet and the OSPF protocol records the data it collects in three tables or databases:

1. Network geography table or network database
2. Routing Table
3. Neighbor table or neighbor database

Then it exchanges this data with other "router" devices in the network.

In large networks, collecting this data and exchanging it with other devices is a matter that requires a great effort from the "router", and to reduce this burden, the OSPF protocol follows two methods:-

The first method is to allocate a “Router” device called the Designated Router DR, which collects data, and then exchanges this data with other “Router” devices.
The second method is by dividing large networks into a group of parts, each part called an area. The OSPF protocol implements its work in three main stages, which are as follows:-

Data Collection

At this stage, the protocol collects various data in two steps as follows:-
The first step: collecting data on the Topology networks.
In this step, the protocol collects data about the networks connected to it and records it in its database.
The second step: collecting data on the neighborhood, or discovering the neighborhood.

In this step, the protocol collects data about the surrounding “Router” devices and identifies them, establishes a relationship with them to determine which of them will exchange data with him in the next stage and which of them will not, and records this data in the “Neighborhood” table (Neighbor Table).

Data Exchange

At this stage, the protocol exchanges data with the surrounding devices according to certain conditions. It sends the data it collected to it and receives the data is collected from it.

Best Path Selection

At this stage, the protocol examines the data it has collected and chooses the best paths for each network according to special criteria, and records these paths in its routing table.