Final answer:
The OSPFv3 router ID is identified by the highest IPv4 address on an active interface, or through a manually set router ID if no IPv4 address is present.
Step-by-step explanation:
The router ID for an OSPFv3 router is determined by using the highest IPv4 address on any of the router's active interfaces. If there are no IPv4 addresses configured on the router, then OSPFv3 will use the router ID that is manually configured. If neither of these is available, the router will not be able to form OSPFv3 adjacencies until a router ID has been determined, either by configuring an IPv4 address on one of its interfaces or by manually setting a router ID.