Routers initially exchange Type 2 DBD packets, which is an abbreviated list of the sending router’s LSDB and is used by receiving routers to check against the local LSDB.
A Type 3 LSR packet is used by the receiving routers to request more information about an entry in the DBD.
The Type 4 LSU packet is used to reply to an LSR packet.
LSUs are also used to forward OSPF routing updates, such as link changes. Specifically, an LSU packet can contain 11 different types of OSPFv2 LSAs, as shown in the figure. OSPFv3 renamed several of these LSAs and also contains two additional LSAs.
Note: The difference between the LSU and LSA terms can sometimes be confusing because these terms are often used interchangeably. However, an LSU contains one or more LSAs.