In this lesson, I’ll explain the different types of LSAs (Link-State Advertisements) used in OSPF. LSAs are the building blocks of the OSPF link-state database, which allows routers to understand the network topology. There are multiple types of LSAs, each serving a specific purpose in the OSPF protocol. Let’s dive into the details.

LSA Type 1: Router LSA

Definition: Type 1 LSAs, also called Router LSAs, are generated by every OSPF-enabled router to describe its connected interfaces and their states. These LSAs allow peer routers to understand which networks are connected to a given router.

Key Characteristics:

• Contains a list of all the router’s directly connected links and their associated states.
• Stays within the same OSPF area — it does not cross area boundaries in a multi-area OSPF setup.

 

In the example above, you can see Router LSAs showing link states for Router 1 in Area 0.

LSA Type 2: Network LSA

Definition: Type 2 LSAs, also known as Network LSAs, are generated by the Designated Router (DR) in a broadcast or non-broadcast multi-access (NBMA) network. They describe the subnet and the routers connected to it.

Key Characteristics:

• Generated only by the DR in a multi-access network.
• Includes the subnet and the list of routers connected to that subnet.
• Stays within the same OSPF area — does not propagate to other areas.

 

The output shows a Network LSA sent by the DR for a broadcast network in Area 0.

 

LSA Type 3: Summary LSA

Definition: Type 3 LSAs, or Summary LSAs, are generated by Area Border Routers (ABRs) to advertise networks from one area to another. These LSAs provide routing information about inter-area networks.

Key Characteristics:

• Generated by ABRs.
• Facilitate communication between areas in a multi-area OSPF design.
• Contain routing information about networks in other areas.

The above output shows a Summary LSA generated by the ABR for networks in Area 1.

LSA Type 4: ASBR Summary LSA

Definition: Type 4 LSAs, or ASBR Summary LSAs, are also generated by ABRs. They describe how to reach an Autonomous System Boundary Router (ASBR) from other areas.

Key Characteristics:

• Generated by ABRs to advertise the location of an ASBR.
• Provide information needed for inter-domain routing.

 

The ASBR Summary LSA provides details about how to reach the ASBR.

 

LSA Type 5: External LSA

Definition: Type 5 LSAs, or External LSAs, are generated by ASBRs to advertise external routes (routes outside the OSPF domain, such as those learned from BGP).

Key Characteristics:

• Propagated throughout the OSPF domain (except stub and NSSA areas).
• Provide routing information about external networks.

 

The output displays an External LSA advertising a default route from an ASBR.

 

LSA Type 6: Group Membership LSA

Definition: Type 6 LSAs, or Group Membership LSAs, are used in OSPF multicast extensions. These LSAs are primarily utilized to support multicast OSPF (MOSPF) by carrying information about multicast group memberships.

Key Characteristics:

• Generated by multicast-capable routers to advertise multicast group memberships.
• Propagates within a single area (does not cross area boundaries).
• Typically not seen in standard OSPF configurations unless MOSPF is implemented.

 

The output shows a Group Membership LSA advertising multicast group membership information in Area 0.

 

LSA Type 7: NSSA External LSA

Definition: Type 7 LSAs, or NSSA External LSAs, are generated by routers in Not-So-Stubby Areas (NSSAs) to describe external routes. They are similar to Type 5 LSAs but are limited to NSSAs and are translated into Type 5 LSAs by the NSSA Area Border Router (ABR) when necessary.

Key Characteristics:

• Generated by ASBRs within an NSSA to advertise external routes.
• Propagated only within the NSSA and translated to Type 5 LSAs by the ABR when leaving the NSSA.
• Enables NSSA areas to allow external routes without becoming a full-fledged OSPF backbone area.

The output displays an NSSA External LSA advertising an external route within an NSSA.

Conclusion

In this lesson, I’ve explained the different types of LSAs in OSPF and their respective functions. Each LSA type serves a specific purpose in maintaining the OSPF routing topology. Understanding these LSAs is essential for designing and troubleshooting OSPF networks effectively.

I hope this lecture was informative for you, and I’ll see you in the upcoming lecture.