When configuring OSPF.

In Junos OS, a physical or logical interface is typically associated with a single OSPF area. However, there are specific design requirements—such as migrating an interface from one area to another without downtime or creating specific transit topologies—where an interface needs to exist in two areas simultaneously.

Multi-area Adjacency (Option B):The secondary command under the OSPF interface hierarchy allows an interface to be part of asecondary areain addition to its primary area.

Protocol Behavior:When an interface is configured as secondary, the router sends and receives OSPF Hello packets for both the primary and secondary areas on that single link. This allows the router to form adjacencies with neighbors in both areas over the same physical connection.

Use Case:This is frequently used in hub-and-spoke topologies or during network restructuring to ensure reachability is maintained while changing area boundaries. It effectively allows a "logical" split of the interface at the OSPF protocol level.