LSDB

LSDB stands for Link-State Database, a core component of link-state routing protocols such as OSPF and IS-IS. It stores topology information about the network as learned through link-state advertisements or link-state packets. Each router maintains its own LSDB, and in most protocols there is a separate LSDB per area (OSPF) or per level (IS-IS). LSAs or LSPs are flooded throughout the network to ensure all routers within the same scope share a consistent view of the topology. The LSDB contains information about network links, their costs, statuses, and the reachability of networks and endpoints; it includes entries for each link and for each router or area, depending on the protocol.

In OSPF, the LSDB is populated by specific LSA types: Router-LSAs (type 1), Network-LSAs (type 2), Summary-LSAs (type 3), and AS-External-LSAs (type 5), among others. IS-IS uses Link-State PDUs (LSPs) to convey similar topology information within a level. Each LSA/LSP carries a sequence number and an age, and aging mechanisms ensure stale information is discarded. When topology changes occur, new LSAs/LSPs are flooded, and old entries are refreshed or removed.

The primary purpose of the LSDB is to support the Shortest Path First (SPF) calculation. The router runs SPF on the LSDB to derive a topology tree, which then determines the routing decisions and populates the forwarding information base or routing table. The LSDB is distinct from the actual data-forwarding table; it is a representation of network topology, not of individual routes. Convergence depends on consistent flooding, timely refreshes, and proper aging of LSAs/LSPs across the routing domain.