What value does a router use for the OSPF router ID in the absence of a loopback interface?

C.

In OSPF (Open Shortest Path First) protocol, each router in an OSPF domain is identified by a unique Router ID (RID), which is a 32-bit number represented in dotted decimal format. OSPF uses the RID to identify a router and as a tie-breaker in case of two routers having equal metric values.

By default, the OSPF router ID is selected based on the highest IP address configured on any loopback interface. If no loopback interfaces are configured, the router ID is selected based on the highest IP address configured on any active interface. If all interfaces have the same IP address or no IP address is configured on any interface, the OSPF router ID is selected based on the highest IP address among the interfaces that are up.

If all OSPF routers in a single area are configured with the same priority value, the router ID will be selected based on the above criteria mentioned. None of the answer options A, B, C, or D specifies the correct criterion for the router ID selection.

Therefore, the correct answer is option E: the priority value until a loopback interface is configured. This means that the router ID will be selected based on the router's OSPF priority until a loopback interface is configured. OSPF priority is a value between 0 and 255 that is used in the election of the designated router (DR) and backup designated router (BDR) in a multiaccess network. The router with the highest priority value will become the DR, and the router with the second-highest priority value will become the BDR. In case of a tie, the router with the highest RID will become the DR, and the router with the second-highest RID will become the BDR.

Therefore, if all routers in a single OSPF area have the same priority value, the router with the highest priority value will become the DR, and its RID will be used as the OSPF router ID for all routers in the area until a loopback interface is configured.