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Cisco CCNA 3.4
TechKnowSurge Catalog Interactives OSPF Cost Calculator
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OSPF Cost Calculator

Reference bandwidth over interface bandwidth: work out OSPF costs and which path wins.

⚑ Complete this interactive to capture a CTF flag worth 5 points.

About this interactive

OSPF's metric is cost: **reference bandwidth / interface bandwidth**, so faster links cost less. A path costs the sum of its links, and the lowest-cost path goes into the routing table; equal-cost paths are load-balanced. The default reference is 100 Mbps, from slower days. A cost cannot be a fraction or lower than 1, so 100 Mbps, 1 Gbps and 10 Gbps links all cost 1, and OSPF cannot tell them apart. The lesson's fix, recommended over setting bandwidths or costs by hand on each interface, is to raise the reference bandwidth. At 100,000 Mbps, links of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps cost 10,000, 1,000, 100 and 10. A loopback costs 1 by default.

What you'll learn

Aligned to

Cisco CCNA
3.4 Configure and verify single area OSPFv2

Key terms

OSPF Cost
A metric assigned to each router interface, calculated by dividing the reference bandwidth by the interface bandwidth; lower cost indicates a preferred path.
Reference Bandwidth
A configurable value used in the OSPF cost formula as the numerator; the default is 100 Mbps, which must be increased to differentiate between FastEthernet, GigabitEthernet, and 10-Gigabit interfaces.
OSPF Reference Bandwidth
A configurable value used as the numerator in OSPF cost calculation; adjusting it allows accurate cost differentiation among high-speed interfaces.
Equal-Cost Load Balancing
A routing feature that distributes traffic across multiple paths that share the same administrative distance and metric.

Topics

Ospf Ospf Cost Reference Bandwidth Metric Spf Interactive Calculation

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