Point-to-Point Networks: On point-to-point networks (like a direct link between two routers), OSPF can use unicast to send updates, as there is only one possible neighbor at the other end of the link. Demand Circuits: OSPF can use unicast for demand circuits, which are used to minimize the amount of OSPF traffic over a link. This is particularly useful in scenarios where OSPF traffic needs to be reduced to save bandwidth or minimize charges on a per-usage basis. Virtual Links: When OSPF virtual links are configured to connect non-contiguous areas to the backbone area (Area 0), OSPF may use unicast for communication over these virtual links. Unicast Replies to Database Description Requests: In some implementations, when a router requests specific link-state advertisements (LSAs) after receiving a database description, the responding OSPF router may use unicast to send the requested LSAs.
