OSPF LSA Types Explained (1, 2, 3, 4, 5, 7)
OSPF builds its map from Link-State Advertisements (LSAs) — each type carries a different piece of the topology. Knowing the common types (1, 2, 3, 4, 5 and 7) and which router originates each is essential for reading an OSPF database and troubleshooting.
The common LSA types
| Type | Name | Generated by | Describes |
|---|---|---|---|
| 1 | Router | Every router | Its own links, within an area |
| 2 | Network | DR | Routers on a multi-access segment |
| 3 | Summary | ABR | Inter-area routes |
| 4 | ASBR-Summary | ABR | How to reach an ASBR |
| 5 | External | ASBR | Routes redistributed into OSPF |
| 7 | NSSA External | ASBR in NSSA | Externals inside an NSSA |
Why it matters
When you run show ip ospf database, you're reading these LSAs. Type 5 externals being blocked is exactly what makes a stub area lean; Type 7 exists so NSSAs can still import externals. This is core CCNP ENCOR knowledge, built on CCNA OSPF fundamentals.
Frequently asked questions
What is an OSPF LSA?
A Link-State Advertisement — a message that describes a piece of the network topology. Routers exchange LSAs to build a common map.
Which router generates a Type 2 (Network) LSA?
The Designated Router (DR) on a multi-access segment, describing the routers attached to that segment.
What is a Type 5 LSA?
An External LSA generated by an ASBR to advertise routes redistributed into OSPF from another source; it's flooded through most of the OSPF domain.
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