OSPF LSA Types Tutorial

OSPF builds its Link State Database (LSDB) out of Link State Advertisements (LSAs), so understanding the different LSA types is really the key to understanding how OSPF actually works under the hood. The example topology used throughout this tutorial includes internal routers (routers that sit entirely within a single area), Area Border Routers or ABRs (routers that connect two or more areas together), and the backbone area, Area 0, which every other area must connect to directly, unless a virtual link is used instead.

OSPF actually defines 11 LSA types in total, but in practice only a handful of them matter day to day. This tutorial covers Types 1 through 5, plus Type 7; Type 6 isn’t supported by Cisco at all, and Types 8 through 11 aren’t relevant here.

LSA Type 1 — Router LSA

Every OSPF router generates exactly one Type 1 LSA, describing its own active interfaces, their IP addresses, its neighbors, and the cost to reach each of them. A Router LSA never leaves the area it was generated in — it does not cross an ABR into another area.