Which OSPF network type is enabled on Ethernet and FDDI interfaces by default?

The correct choice refers to the default OSPF network type for Ethernet and FDDI interfaces, which is Broadcast. In OSPF (Open Shortest Path First), the Broadcast network type is designed for multi-access networks where all devices can communicate directly with each other. Ethernet and FDDI meet these criteria, allowing for the use of a Broadcast designation to efficiently manage OSPF routing.

In Broadcast networks, OSPF utilizes a DR (Designated Router) and BDR (Backup Designated Router) to minimize the amount of OSPF traffic and streamline the routing information exchange among routers. This reduces the overall resource usage and keeps the OSPF process efficient in environments where many routers are interconnected.

Other network types mentioned have specific applications that do not apply to standard Ethernet and FDDI configurations. For example, point-to-point networks are used in direct connections between two routers, while non-broadcast networks like Frame Relay require a different configuration due to their inability to support broadcasts. Mesh is not a standard OSPF network type; instead, it refers to a topology. Thus, Broadcast is the correct answer for Ethernet and FDDI interfaces in the context of OSPF default configurations.