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Routing & Switching · Foundational

Router ID (RID)

A 32-bit identifier, written as an IPv4 address, that uniquely identifies a router within OSPF, EIGRP or BGP.

Definition

Cisco selects the router ID from a manual configuration, otherwise the highest loopback IPv4 address, otherwise the highest active physical interface address. In OSPF the RID is chosen when the process starts and changes require clear ip ospf process. Duplicate router IDs cause adjacency and LSDB problems.

Example

router-id 1.1.1.1 under router ospf 1

Exam tip

Know the RID selection order: manual, highest loopback, highest physical interface.

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