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In OSPF networking, an LSA is a link-state advertisement: information routers use to describe network topology or reachability. This guide covers the principal OSPFv2 LSA types. “LSA” can also mean Google Local Services Ads, a separate advertising product, but that is not the subject here.
What OSPF LSAs tell routers
OSPF routers exchange LSAs to build and maintain a view of network topology and routes. An LSA’s type identifies the information it carries; its flooding scope determines where that information can travel. A router does not necessarily receive every LSA in the OSPF domain: the area design and LSA type both matter. Cisco’s OSPFv2 configuration guide documents the principal types summarized below.
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OSPFv2 LSA types at a glance
| Type | Common name | What it communicates | Origin and scope |
|---|---|---|---|
| 1 | Router LSA | A router’s links and associated costs within an area. | Originated by routers; describes intra-area topology. |
| 2 | Network LSA | A multiaccess network segment and the routers attached to it. | Associated with the designated router for that segment; describes the network within its area. |
| 3 | Summary LSA (network summary) | Reachability to networks in another area. | Advertised by an area border router (ABR) as inter-area network information. |
| 4 | ASBR Summary LSA | How to reach an autonomous system boundary router (ASBR) from another area. | Advertised by an ABR; helps routers resolve an ASBR referenced by external advertisements. |
| 5 | AS-External LSA | Routes redistributed into OSPF from outside the OSPF domain. | Originated by an ASBR and distributed through areas that permit external LSAs. |
| 7 | NSSA External LSA | External route information within a Not-So-Stubby Area (NSSA). | Originated by an ASBR in the NSSA; an NSSA ABR can translate it into a Type 5 for propagation outside the area, subject to the applicable configuration and translation rules. |
How to distinguish the LSAs that are easy to confuse
Type 1 versus Type 2: router links and shared networks
A Type 1 LSA describes a router’s links and costs inside its area. A Type 2 LSA describes a multiaccess segment and the routers attached to it. In other words, one represents a router’s perspective on its links; the other represents the shared network segment.
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Type 3 versus Type 4: network reachability and ASBR reachability
A Type 3 advertises reachability to networks across area boundaries. A Type 4 advertises how to reach an ASBR from another area. They solve different problems: Type 3 concerns destination networks, while Type 4 supplies reachability to the router that may be originating external routes.
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Type 5 versus Type 7: external routes and NSSA handling
Type 5 is the familiar AS-external advertisement for routes brought into OSPF from outside the domain. An NSSA uses Type 7 to carry external route information within that area. In the documented NSSA process, the NSSA ASBR originates Type 7 and the NSSA ABR translates it into Type 5 for wider distribution. Translation depends on NSSA configuration and rules; it is not safe to assume every Type 7 is always translated. See Cisco’s NSSA explanation.
Why area type changes which LSAs appear
OSPF areas control the propagation of routing information. Stub-area designs restrict certain summary or external advertisements, while an NSSA permits external route injection using Type 7. The area’s policy and configuration therefore affect which LSAs a router sees and where they are flooded. Cisco’s area guide discusses stub restrictions and Type 7 translation in NSSAs.
- Router and network LSAs: describe topology within an area.
- Summary LSAs: carry inter-area network reachability or, for Type 4, reachability to an ASBR.
- External LSAs: carry routes redistributed into OSPF, with Type 5 or the NSSA-specific Type 7 mechanism depending on the area context.
OSPFv3 is not the same LSA catalogue
The table above is a teaching summary of principal OSPFv2 types, not a complete guide to OSPFv3. OSPFv3 includes version-specific LSA behavior, including types for link and intra-area prefix information. Do not apply the OSPFv2 table as a complete OSPFv3 reference. Cisco’s OSPFv3 NSSA documentation illustrates those version differences.
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