Every OSPF (Open Shortest Path First) router needs a router ID (RID), a 32-bit value written like an IPv4 address that identifies it in the link-state database (LSDB), in neighbour tables and in DR (designated router) elections. It does not have to be a reachable address, but it must be unique within the OSPF domain. Knowing how the RID is chosen, and the two ways to enable OSPF on interfaces, lets you read and write almost any CCNA OSPF configuration.
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