What is LSDB?

The LSDB (Link State Database) is the set of LSAs a link-state router holds for an area: its map of the network, which SPF runs over.

In OSPF, every router describes its own links in LSAs and floods them to every other router in the area. Each router stores what it receives in its LSDB. Each LSA is identified by its type, link state ID and advertising router, and carries a sequence number, an age and a checksum, so a router can tell whether a copy it receives is newer than the one it holds. Originators refresh their LSAs every 30 minutes, and an LSA whose age reaches one hour (MaxAge) is flushed. When two routers form an adjacency, they compare database summaries with DBD packets, request what they're missing, and only reach Full once their databases match. The result is that every router in an area holds an identical LSDB. Each then runs Dijkstra's SPF algorithm with itself as the root, so the routing tables differ even though the map is the same.

An ABR keeps one LSDB per attached area, which is the main reason areas exist: a smaller database means less memory, less flooding and quicker SPF runs. In FRR and Cisco-style CLIs, show ip ospf database lists the LSDB as a table of LSA headers, and adding an LSA type shows the contents. When routes look wrong, the LSDB is where to start: if an expected LSA is missing, the problem is an adjacency or flooding; if it is present but the route is wrong, look at costs or the SPF result. IS-IS keeps an equivalent database built from LSPs. See LSA and SPF; the OSPF Fundamentals lesson explains how the LSDB is built and used.