What is LSA?

An LSA (Link State Advertisement) is a unit of OSPF topology information, such as one router's list of links, that is flooded to every router in an area.

Each OSPF router describes its piece of the network in LSAs and floods them to its neighbours, which pass them on, until every router in the area holds an identical copy in its LSDB. LSAs are carried inside Link State Update (LSU) packets and every LSA received must be acknowledged, so flooding is reliable. Every LSA has a common header that includes its type, a link-state ID, the advertising router, a sequence number and an age. The sequence number lets routers tell a newer copy from an older one, and the age counts upwards: routers refresh their own LSAs every 30 minutes, and an LSA that reaches an hour old is flushed. The main types in OSPFv2 are:

• Type 1, Router: a router's own links and their costs inside the area.
• Type 2, Network: originated by the DR, listing routers on a multi-access segment.
• Type 3, Summary: prefixes from another area, sent by an ABR.
• Type 4, ASBR Summary: how to reach an ASBR in another area.
• Type 5, External: routes redistributed into OSPF by an ASBR.
• Type 7, NSSA External: external routes inside an NSSA, converted to Type 5 by the ABR.

Reading the LSA database is the main way to troubleshoot OSPF, with commands such as show ip ospf database in FRR. Area types such as stub and NSSA are really rules about which LSA types are allowed in, so knowing the types explains them. A common confusion is treating LSAs as routes: they describe the topology, and each router's SPF calculation turns them into routes. The OSPF Fundamentals lesson covers flooding and every LSA type with a lab.