Network Design and Operations
Using SNMP, a router interface's 64-bit counter of received bytes is read every 5 minutes (300 seconds). The previous reading was 1,200,000,000 and the current one is 4,950,000,000. Which is closest to the average receive rate over these 5 minutes?
- A12.5 Mbps
- B100 Mbps
- C500 Mbps
Show answer and explanation
Answer: B. 100 Mbps
The interface received 4,950,000,000 − 1,200,000,000 = 3,750,000,000 bytes in 5 minutes, or 3.75×10⁹ × 8 = 3×10¹⁰ bits. Dividing by 300 seconds gives 1×10⁸ bit/s, or 100 Mbps. In SNMP, the managing side (the manager) queries MIB values from the agent on the device and, as here, works out traffic from differences between counter readings to draw graphs. A 32-bit counter wraps back to 0 after 2^32 ≈ 4.29×10⁹ bytes, which at 100 Mbps takes only about 344 seconds, so 64-bit counters are used for fast interfaces.
- A. This value does not convert bytes to bits (it is 12.5 MB/s).
- B. Correct. 3.75×10⁹ × 8 ÷ 300 = 1×10⁸ bit/s.
- C. This divides the difference by 60 seconds instead of 300.