OSPFv2 (IPv4) and OSPFv3 (IPv6) share many fundamental concepts, such as the SPF algorithm and the use of areas, but they have distinct differences in how they handle prefix information and LSA headers in Junos OS.

Backbone Requirement (Option C):Both OSPFv2 and OSPFv3 utilize a hierarchical structure. In both versions, all areas must connect to a centralArea 0 (the backbone)to allow for inter-area routing. If an area is not physically connected to Area 0, a virtual link must be established.

Flooding Scope (Option D):One of the significant architectural changes in OSPFv3 is the inclusion of theflooding scopedirectly into the LSA header. In OSPFv3, the LS Type field is expanded to 16 bits, where the top three bits define the flooding scope (e.g., Link-local, Area, or AS scope). This allows routers to understand how far an LSA should be propagated without needing to understand the specific LSA type itself.

LSA Prefix Information (Option A):This statement is misleading. In OSPFv2, Router and Network LSAs (Type 1 and 2) carry both topology and prefix information. However, a key design goal of OSPFv3 was toseparatetopology from prefix information. In OSPFv3, Type 1 and 2 LSAs only describe the topology, while prefixes are carried in new LSA types:Intra-Area-Prefix-LSAs(Type 9) andLink-LSAs(Type 8).

Processing (Option B):OSPFv2 is processed "per-subnet" because it forms adjacencies based on IPv4 subnets. OSPFv3 is processed"per-link", meaning it forms adjacencies between interfaces on the same physical link regardless of whether they share a common IPv6 prefix.