If no router ID was manually configured, what would a router use as its OSPF router ID?

When no router ID is manually configured in an OSPF (Open Shortest Path First) environment, the router automatically selects its router ID based on the highest IP address assigned to its active interfaces. This process ensures that each router has a unique identifier in the OSPF domain, which is crucial for the protocol's operation and routing decision-making.

Using the highest IP address among all active interfaces helps to avoid conflicts and ensures that each router can be uniquely identified. OSPF uses this router ID to form adjacencies with other OSPF routers and to maintain a consistent view of the routing topology. By choosing the highest IP address, OSPF also facilitates a deterministic process for selecting the router ID, leading to predictable behavior in network configurations.

This default behavior streamlines the setup of OSPF, reducing the manual configuration needed and minimizing the potential for human error in larger networks.