Which topology does OSPF use on Frame Relay and X.25 interfaces by default?

OSPF (Open Shortest Path First) uses non-broadcast multi-access (NBMA) topology by default on Frame Relay and X.25 interfaces. This is due to the nature of these types of networks, where data is transmitted over a shared medium but the devices do not have the capability to send broadcast packets to each other.

In NBMA environments, OSPF needs to be configured in a way that allows routers to discover neighbors without relying on broadcast communication. As a result, OSPF considers each link between two routers as a point-to-point connection, but it treats the overall network as non-broadcast, requiring manual configuration of neighbors and OSPF settings.

This topology choice allows for efficient routing and better handling of traffic in environments where not all devices can communicate freely with one another, as is the case with Frame Relay and X.25 technologies. The approach helps maintain the advantages of OSPF while adapting to the constraints imposed by the underlying network infrastructure.