LSA types in OSPF

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.

R1# show ospf database router

Router Link States (Area 0.0.0.0)

Link ID ADV Router Age Seq# Checksum Link count

10.10.10.1 10.10.10.1 120 0x8000002D 0xF39C 4

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.

R1# show ospf database network

Network Link States (Area 0.0.0.0)

Link ID ADV Router Age Seq# Checksum

10.10.20.1 10.10.10.2 200 0x80000015 0x9F24

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.

R2# show ospf database summary

Summary Net Link States (Area 0.0.0.1)

Link ID ADV Router Age Seq# Checksum

192.168.1.0 10.10.10.2 230 0x80000018 0x8A31

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: