When a router has been configured with a loopback address, which of the following determines the OSPF
router ID?

A.
The highest MAC address assigned to a physical interface on the router

B.
The lowest priority of a physical interface on the router

C.
The lowest IP address assigned to a physical interface on the router

D.
The highest IP address assigned to a loopback interface on the router

Explanation:
Routers configured with OSPF must be assigned a router ID (RID), which is an IP address unique across the
entire OSPF autonomous system. The RID can be assigned manually with the router-id command, or it will be
determined automatically by OSPF. If the RID has not been manually assigned, then OSPF will use the highest
numerical IP address of a loopback interface on the local router. If there are no configured loopback interfaces,
then the RID will be determined by the highest numerical IP address on an active physical interface. The
sequence for determining the RID is as follows:
1. Any address manually configured with the router-id command
2. The highest IP address on a loopback interface
3. The highest IP address on an active physical interface
Either of the first two options would be a recommended best practice, since they each offer fault tolerance to
the RID. If the RID is determined by a physical interface IP address, then the entire OSPF routing process is
bound to an interface that could become unplugged or go down due to network reasons.
Loopback interfaces remain operational unless they are manually shut down. Loopback interfaces are
configured as follows:
Router(config)# interface loopback0
Router(config-if)# ip address 192.168.1.254 255.255.255.255
The highest media access control (MAC) address assigned to a physical interface on the router is not used. IP
addresses are used for the determination of the router ID.
Priorities are not used to determine the OSPF router ID. Priorities are used by OSPF to influence the election of
the designated router (DR) and backup designated router (BDR) on a multi-access segment.
Router IDs are determined by the highest IP address on a loopback or physical interface, not the lowest.
Objective:
Routing Fundamentals
Sub-Objective:
Configure, verify, and troubleshoot single area and multi-area OSPFv2 for IPv4 (excluding authentication,
filtering, manual summarization, redistribution, stub, virtual-link, and LSAs)