In which OSPF state do routers send LSR messages to request LSAs from their neighbor?

In the OSPF (Open Shortest Path First) routing protocol, the state in which routers send Link State Request (LSR) messages to request Link State Advertisements (LSAs) from their neighbors is the Loading state. This state occurs after routers have exchanged their Database Description (DBD) packets, which contains summaries of the LSAs they have.

During the Loading state, a router that has determined that it has outdated or missing LSAs will specifically send LSR messages to its neighbor. The purpose of these messages is to request the necessary LSAs to populate its link-state database and ensure that its routing information is up to date. Once the routers successfully complete this exchange of needed LSAs and have received all necessary information, they transition into the Full state, indicating a complete adjacency and synchronized databases.

Other states, such as Exstart and Exchange, involve the initial setup and negotiation for the database exchange but do not specifically involve sending LSRs to request LSAs.