OSPF

OSPF Cost and Path Selection

Learn how OSPF assigns interface costs, builds shortest paths, compares external routes, and influences SPF timing and redundancy decisions.

Intermediate12 min read
OSPF lessonNetworkingPractice

UNTIMED COACHING

Practice before the pressure

Use feedback to correct the model, not merely memorize an option position.

GUIDED PRACTICE

Practice the lesson questions

Answer normal lesson questions without a timer. Every answer includes an explanation, and incorrect answers can be tried again before continuing.

CONCEPT MODELS

See the lesson as a system

Use these visual guides to connect the key ideas before answering the questions.

From Interface Cost to Next HopOSPF converts interface values into path metrics, builds a shortest-path tree, and selects next hops.
From Interface Cost to Next HopA flow from interface bandwidth and reference bandwidth through costs and path comparison to a routing-table next hop.
Reference bandwidthSets the calculation basis
Interface bandwidthDescribes interface capacity
Interface costDerived metric for comparison
Candidate pathsAccumulate link costs
Shortest-Path TreeA router roots a tree at itself and uses accumulated metrics to determine routing-table next hops.
Shortest-Path TreeA router root leads to candidate paths, a least-cost tree, and a selected next hop.
Local routerRoots the shortest-path tree
Candidate routesOffer different accumulated cost
Shortest-path treeRecords least-cost relationships
Routing-table next hopUses the least-cost path
External Metric ComparisonType 1 combines external and internal components, while equal Type 2 values use forwarding-address distances.
External Metric ComparisonTwo branches compare Type 1 addition with Type 2 forwarding-address distance.
External componentMeasures outside route portion
Internal componentMeasures internal route portion
Type 1 totalAdds both cost components
Equal Type 2Compares forwarding distances