Cisco CCNA (200-301)IP ConnectivityHard
An OSPF network uses the default reference bandwidth. R1 connects to R2 via a Fast Ethernet interface (100 Mbps) and R2 connects to R3 via a serial T1 interface (1544 kbps). What is the total OSPF cost from R1 to R3 across both links (rounded to the nearest whole number, using cost = reference bandwidth / interface bandwidth)?
- A2
- B164
- C65
- D101
Show answer & explanationAnswer & explanation
Correct answer: C. 65
Default OSPF reference bandwidth is 100 Mbps (100,000 kbps). Fast Ethernet cost = 100,000/100,000 = 1. T1 cost = 100,000/1544 = 64.77, rounded down to 64 by IOS. Total path cost = 1 + 64 = 65.
Why the other options are wrong
- A. This is only the Fast Ethernet segment's cost, ignoring the T1 link.
- B. This overstates the T1 cost; the correct calculation yields 64, not a higher value.
- D. This value does not match the actual per-interface cost calculation using default reference bandwidth.
OSPF Cost Calculation
OSPF calculates interface cost as reference bandwidth divided by interface bandwidth; the total path cost is the sum of costs of all outgoing interfaces along the path.
- Default reference bandwidth = 100 Mbps (100,000 kbps)
- Formula: cost = reference bandwidth (kbps) / interface bandwidth (kbps)
- IOS truncates the cost to an integer (rounds down)
Memory trick: Divide the reference by the pipe size to get the toll for each road.