- a loopback interface that is configured with the highest IP address on the router
- any IP address that is configured using the router-id command
- the highest active interface IP that is configured on the router
- the highest active interface that participates in the routing process because of a specifically configured network statement
Explanation: The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If a router ID that is configured through the router-id command is not available, OSPF routers next use the highest IP address available on a loopback interface, as loopbacks used as router IDs are also not routable addresses. Lacking either of these alternatives, an OSPF router will use the highest IP address from its active physical interfaces.
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