Network Design and Operations
In OSPF, router R1 has three paths to a network beyond R4. Path A uses a 10 Gbps link R1–R5 and a 1 Gbps link R5–R4; path B is a direct 100 Mbps link R1–R4; and path C uses 1 Gbps links R1–R2, R2–R3 and R3–R4. The cost of each link is "reference bandwidth 10 Gbps ÷ link bandwidth", and the cost beyond R4 is the same for all paths. Which path does R1 choose?
- APath A
- BPath B
- CPath C
Show answer and explanation
Answer: A. Path A
The costs are 10 ÷ 10 = 1 for a 10 Gbps link, 10 ÷ 1 = 10 for 1 Gbps and 10 ÷ 0.1 = 100 for 100 Mbps. Path A costs 1 + 10 = 11, path B 100 and path C 10 × 3 = 30, so path A, with the lowest total cost, is chosen. If the reference bandwidth is left at 100 Mbps, long the default on much equipment, every link of 100 Mbps or faster costs 1, giving path A 2, path B 1 and path C 3, and the slowest path, B, is chosen. Networks with fast links therefore set the reference bandwidth to a larger value, the same on every router.
- A. Correct. Its total cost, 1 + 10 = 11, is the lowest.
- B. It has only one link, but its cost of 100 is the highest. RIP, which chooses by hop count, would choose this path.
- C. Its cost of 10 × 3 = 30 is higher than path A's 11.