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Routing & Switching · Intermediate

Shortest Path First Algorithm (SPF)

Dijkstra's algorithm, used by OSPF and IS-IS to compute a loop-free shortest path tree from the LSDB.

Definition

Each router places itself at the root and computes the lowest cumulative cost path to every node. Full SPF runs are triggered by topology (Type 1/2) changes, whereas prefix-only changes can be handled by partial route calculation. SPF throttling timers control how quickly successive runs occur.

Example

timers throttle spf 50 200 5000 under router ospf 1

Exam tip

OSPF interarea routes (Type 3) do not trigger a full SPF; they use distance-vector style calculation.

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