OSPF

In OSFP, the network is divided into regions called "areas," and route information is managed for each area.
The backbone area is the core of the OSPF network, and other areas are placed around it depending on their intended use.

There are four types of OSPF areas:

Devices that belong to an OSPF area are called "OSPF routers" and are identified by a unique "router ID."
OSPF routers are classified into the following three types depending on where they are located:

In OSPF, routers exchange information called Link-State Advertisements (LSAs) to understand the topology both within and outside their areas. The link state is then managed by creating a link state database (LSDB) by combining the received LSAs with its own LSAs. All OSPF routers belonging to the same area maintain the same LSDB and are constantly synchronized. Therefore, even if there is a change in the route information, the changes are synchronized with the LSDB of other OSPF routers, allowing the failure to be detected and communication to be restored using an alternative route.

In OSPF, the following messages are used to search for OSPF routers that belong to the same area and to exchange route information.

The process flow up to obtaining LSA using the above message is as follows:

In OSPF, in a network that supports multicast, a designated router (DR) and a backup designated router (BDR) are selected from among the OSPF routers that belong to that area. The designated router is responsible for advertising any changes to route information to OSPF routers within the area. By eliminating notification of route information changes by other devices, unnecessary LSA exchanges can be reduced. If the designated router goes down, the backup designated router will serve as the designated router.
OSPF routers that are not selected as either the designated router or the backup designated router exchange LSAs only with the designated router and the backup designated router.

For example, in the configuration shown in Figure 3, if there is a change in the route information in Area 1 due to a network outage or other reason, SWX (3) sends an LSU to the designated router SWX (1) and the backup designated router SWX (2) to advertise the change in route information to the backbone area. The designated router that receives the LSU advertises it to other OSPF routers within the area. At this point, the backup designated router does not advertise the LSU.

OSPF routers located within a certain OSPF area are said to be in a "neighboring relationship" when they confirm that they are OSPF routers by sending Hello messages to each other. Also, a relationship in which LSAs are exchanged directly is called an "adjacency." OSPF routers can be classified into neighbors and adjacencies based on these relationships.

There are six types of Link State Advertisements (LSAs), which are used depending on their purpose and type of advertisement:

In OSPF, all areas must be directly connected to the backbone area. However, there are some networks that cannot be directly connected to the backbone area due to their physical configuration. In such cases, you can logically connect to the backbone area by using a virtual link.
For example, if a network is added to an initially constructed OSPF network, it does not necessarily have to be connected directly to the backbone area; it can be connected to the nearest standard area and logically connected to the backbone area using a virtual link.

Figure 4 shows an example of adding Standard Areas 2 and 3 to an OSPF network consisting of a backbone area and Standard Area 1. The backbone area and Standard Area 2 exchange route information via Summary-LSA that passes through Standard Area 1, so that Standard Area 2 is treated as a directly connected point-to-point network when viewed from the backbone area.
Between Standard Area 2 and Standard Area 3, route information is exchanged via the backbone area, but actual communication does not necessarily pass through the backbone area. If calculating the cost of each link determines that the shortest route is one that does not go through the backbone area, communication between Standard Area 2 and Standard Area 3 will only pass through Standard Area 1.