OSPF LSA Types and LSA Flooding Explained

An LSA is a data packet that contains routing information in a format that OSPF running routers understand. OSPF running routers use it to exchange routing information. They also maintain a database to keep track of all exchanged LSAs. This tutorial provides a detailed explanation of LSA and its various types.

What is an LSA?

An LSA (Link State Advertisement) is a data packet that describes a specific part of the OSPF network. There are 11 types of LSA. OSPF routers utilize each type of LSA to exchange different information. Since an LSA describes a part of the OSPF network, each OSPF router learns all LSAs that it needs to function and stores them in the LSDB (Link-State Database) In other words, an LSA is an OSPF data packet that contains some specific information about network topology. The LSDB is the collection of all LSA packets the router knows. Convergence is the state in which all OSPF routers are aware of all LSAs.

LSA flooding

LSA flooding is a process that OSPF routers use to share and learn all required LSAs. During the LSA flooding process, all routers collectively advertise all known LSAs to one another. At the end of this process, every router on the network has all the required LSAs. After this process, if any information changes, the affected router creates an LSA describing the change and floods the LSA into the network. Each recipient validates the LSA update and sends an acknowledgment back to the sending router, confirming receipt of the flooded update.

Based on the network type, OSPF routers use the following addresses to flood LSAs.

• In a point-to-point network, they use the reserved multicast address 224.0.0.5.
• In a broadcast network, they use the multicast address 224.0.0.6.
• In a point-to-multipoint network, they use the adjacent router's unicast IP address.

Types of LSAs

Instead of using the same LSA type for all purposes, OSPF routers use a different kind of LSA for each purpose. They use 11 types of LSAs. The following table describes each type of LSA and its purpose.

The opaque LSA types are designed for application-specific purposes. For example, an application can use them to flood bandwidth information. Opaque LSA types are 9,10, and 11. Each type of LSA has a different flooding scope.

Important LSA types for the CCNA exam

For the CCNA Routing and Switching exam, you should focus only on LSA types 1, 2, and 3. The CCNA exam syllabus does not cover the remaining LSA types.

The following image shows an example OSPF network.

LSA type 1

An LSA type 1 message is known as a router LSA. OSPF routers use LSA type 1 messages to advertise their RID and information about directly connected networks. A RID is the unique ID of the router in the OSPF network. Since all routers in the area require this information about other routers, all OSPF routers generate and advertise LSA type 1 messages. LSA type 1 messages do not cross the area boundary. They remain within the area.

LSA type 2

An LSA type 2 message is known as a network LSA. As mentioned earlier, in an Ethernet/broadcast environment, routers do not share routing information directly. They elect a DR, and then the DR shares the routing information with the other routers. To share routing information, the DR uses LSA type 2 messages. LSA type 2 messages also do not cross the area boundary. They stay within the area.

LSA type 3

An LSA type 3 message is known as the summary LSA. ABR routers use LSA type 3 messages to exchange routing information with the other ABR routers. By default, LSA type 3 messages contain detailed routing information. However, if required, an administrator can instruct the ABR router to summarize the routing information before sharing it with another ABR. For this, the administrator can use the summary command.

Conclusion

OSPF running routers use LSAs to exchange routing information. There are eleven types of LSAs. This tutorial explained these types in detail. Learning these types allows you to understand the OSPF process and concept more effectively.