LSAs are the building blocks of the OSPF LSDB. Individually, they act as database records and provide specific OSPF network details. In combination, they describe the entire topology of an OSPF network or area.

The RFCs for OSPF currently specify up to 11 different LSA types (Figure 1). However, any implementation of multiarea OSPF must support the first five LSAs: LSA 1 to LSA 5 (Figure 2). The focus of this topic is on these first five LSAs.

Each router link is defined as an LSA type. The LSA includes a link ID field that identifies, by network number and mask, the object to which the link connects. Depending on the type, the link ID has different meanings. LSAs differ on how they are generated and propagated within the routing domain.

Note: OSPFv3 includes additional LSA types.