In OSPF, at which state can neighbors exchange their Link State Database (LSDB)?

In OSPF (Open Shortest Path First), neighbors begin exchanging their Link State Database (LSDB) during the Full state. At this point, both routers have fully established their relationships and are ready to share information about the state of their links.

To understand the significance of this state, consider that the Full state indicates that the OSPF adjacency is complete; both routers have successfully exchanged their Hello packets, identified each other as neighbors, and synchronized their LSDBs. This synchronization is crucial for routers to understand the current topology of the network and make informed routing decisions.

In contrast, during the Init state, while a router has received a Hello packet from a neighbor, it hasn't yet established a bi-directional communication link. In the Exstart state, routers negotiate the initial sequence number to be used for LSDB exchanges, but they have not yet exchanged any LSDB data. The 2-way state indicates that routers recognize each other as neighbors but do not imply that the LSDBs have been exchanged; they merely confirm that both peers can communicate.

Thus, the Full state is the correct answer since it is the stage where OSPF neighbors actively exchange their LSDBs and are fully synchronized in terms of network topology information.