CompTIA Network+ (N10-009)Network ImplementationMedium
A network engineer configures OSPF on a router using the default reference bandwidth of 100Mbps. A path to a destination traverses a 1Gbps link, a 100Mbps link, and a 10Mbps link in sequence. What is the total OSPF cost of this path?
- A111
- B1110
- C3
- D12
Show answer & explanationAnswer & explanation
Correct answer: D. 12
OSPF cost per link equals the reference bandwidth divided by the interface bandwidth (both in Mbps), with a minimum cost of 1. The 1Gbps link yields 100/1000=0.1, rounded up to a minimum cost of 1; the 100Mbps link yields 100/100=1; the 10Mbps link yields 100/10=10. Total cost = 1+1+10 = 12.
Why the other options are wrong
- A. This incorrectly treats cost as additive raw bandwidth values.
- B. This value results from an incorrect formula, not the standard cost calculation.
- C. This ignores the higher cost contributed by the slow 10Mbps link.
OSPF Cost Calculation
OSPF assigns a cost to each interface based on reference bandwidth divided by interface bandwidth; the total path cost is the sum of all outgoing interface costs along the route.
- Default reference bandwidth is 100Mbps (10^8)
- Formula: cost = reference bandwidth / interface bandwidth
- Minimum cost value is 1, even for very fast links
Memory trick: Slower links cost more—OSPF favors the fast lane.