Link State Advertisement in OSPF

In OSPF, LSAs (Link State Advertisement)  are messages sent by routers to advertise information about their local networks and the state of their links to neighboring routers. Each router in an OSPF network maintains a Link State Database (LSDB) that contains a copy of all the LSAs in the network. The LSDB is used to calculate the shortest path to each destination network in the network.There are several types of LSAs used in OSPF, including:

• Router LSA: Generated by each router to advertise information about its directly connected links.
•  Network LSA: Generated by the designated router (DR) on a multi-access network to advertise information about the network.
• Summary LSA: Generated by Area Border Routers (ABRs) to advertise summary information about networks in other areas.
• AS External LSA: Generated by Autonomous System Boundary Routers (ASBRs) to advertise information about networks outside the OSPF domain.
• Type 4 LSA: Generated by the ASBR in the backbone area to advertise the existence of an ASBR in other areas.
• Type 5 LSA: Generated by the ASBR to advertise external routes from the AS into the OSPF domain.
• Type 7 LSA: Used in Not-So-Stubby-Areas (NSSAs) to advertise external routes from the NSSA into the OSPF domain.

In nutshell, LSAs are flooded throughout the OSPF domain to ensure that all routers have the same information about the network topology. Routers use the information in LSAs to calculate the shortest path to each destination network in the network.

OSPF Areas

OSPF divides networks into areas to reduce the amount of routing information that must be exchanged between routers. There are several types of OSPF areas:

• Backbone area (Area 0): This is the central area of an OSPF network and is required for all OSPF networks. It connects all other OSPF areas.
• Standard area: A standard area is any OSPF area that is not the backbone area (Area 0).
• Totally Stub area: In a totally stub area, all external routes, including those that would normally be advertised in the OSPF domain, are replaced by a single default route. This reduces the amount of routing information in the area and makes it easier to manage.
• Not-so-Stubby-Area (NSSA): An NSSA is a type of area in which external routes are allowed, but are translated into internal routes within the NSSA. This allows the area to be connected to an external network while maintaining the benefits of OSPF.
• Totally NSSA: In a totally NSSA, external routes are translated into internal routes within the NSSA, and a default route is injected into the area. This further reduces the amount of routing information in the area and simplifies management.
• Transit area: A transit area is an OSPF area through which traffic must pass in order to reach another OSPF area. It typically connects multiple standard areas to the backbone area.

Each OSPF area has its own topology database, which contains information about the routers and networks within that area. This allows OSPF to maintain separate routing tables for each area and reduce the amount of routing information that must be exchanged between routers.

How OSPF Works

OSPF (Open Shortest Path First) is a link-state routing protocol that works by building a complete topology of the network. OSPF routers exchange information about their directly connected links and build a map of the entire network. This map is then used to calculate the shortest path to a destination, making OSPF an efficient routing protocol.

1. OSPF routers discover their neighbors: OSPF routers use hello messages to discover their neighbors and establish adjacency. Routers exchange hello messages to determine if they are on the same network segment and agree on the OSPF parameters.
2. OSPF routers exchange link-state information: OSPF routers exchange link-state advertisements (LSAs) that describe the state of their directly connected links. These LSAs are flooded throughout the network to build a complete topology map of the network.
3. OSPF routers calculate the shortest path to a destination: OSPF routers use the received LSAs to build a shortest path tree of the network, using the Dijkstra algorithm to calculate the shortest path to each destination.
4. OSPF routers select the best path to a destination: OSPF routers select the best path to a destination based on the path’s total cost, which is calculated as the sum of the link costs along the path. The router then forwards the packet to the next hop router along the chosen path.

In summary, OSPF works by building a complete topology map of the network, calculating the shortest path to a destination using the Dijkstra algorithm, and selecting the best path based on the path’s total cost. OSPF routers maintain the network topology by continually monitoring the network and updating their LSAs, and converge quickly when there is a change in the network.

Pre-requisites: Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF)  is a routing protocol that exchanges routing information within an autonomous system (AS). it is a link-state protocol that provides a more efficient and scalable way of routing traffic compared to distance vector protocols like RIP.it uses a hierarchical network design with areas, where each area has its own link-state database and all routers within an area have identical information about that area. Dijkstra algorithmis used  to calculate the shortest path between two points in a network, based on the link-state information available.