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Routing & Switching · Foundational

Open Shortest Path First (OSPF)

An open-standard link-state IGP that uses areas, LSAs and Dijkstra's SPF algorithm with a cost metric.

Definition

OSPF (OSPFv2 for IPv4, RFC 2328) runs directly over IP protocol 89 and multicasts to 224.0.0.5 (all OSPF routers) and 224.0.0.6 (DR/BDR). Routers form adjacencies, flood LSAs within areas and compute shortest paths using interface cost. A two-level hierarchy with backbone area 0 limits flooding and SPF scope. Cisco AD is 110.

Example

router ospf 1 / network 10.0.0.0 0.255.255.255 area 0

Exam tip

OSPF neighbors must match area, subnet, hello/dead timers, authentication, stub flags and (for full adjacency) MTU.

Don't confuse it with

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