CAS-001 · Question #70
The security manager is in the process of writing a business case to replace a legacy secure web gateway so as to meet an availability requirement of 99.9% service availability. According to the…
The correct answer is A. The firewall will meet the availability requirement because availability will be 99.98%. Availability equals MTBF divided by (MTBF + MTTR), yielding 99.98% for this firewall, which exceeds the 99.9% requirement.
Question
The security manager is in the process of writing a business case to replace a legacy secure web gateway so as to meet an availability requirement of 99.9% service availability. According to the vendor, the newly acquired firewall has been rated with an MTBF of 10,000 hours and has an MTTR of 2 hours. This equates to 1.75 hours per year of downtime. Based on this, which of the following is the MOST accurate statement?
Options
- AThe firewall will meet the availability requirement because availability will be 99.98%.
- BThe firewall will not meet the availability requirement because availability will be 85%.
- CThe firewall will meet the availability requirement because availability will be 99.993%.
- DThe firewall will not meet the availability requirement because availability will be 99.2%.
How the community answered
(36 responses)- A75% (27)
- B6% (2)
- C17% (6)
- D3% (1)
Why each option
Availability equals MTBF divided by (MTBF + MTTR), yielding 99.98% for this firewall, which exceeds the 99.9% requirement.
Applying the standard availability formula - Availability = MTBF / (MTBF + MTTR) - with the given values produces 10,000 / (10,000 + 2) = 10,000 / 10,002 = 0.9998, or 99.98%. This result exceeds the 99.9% service availability requirement, so the firewall satisfies the business case threshold.
An availability of 85% would require MTTR to be a large fraction of the total cycle time and is not produced by dividing 10,000 by 10,002; this figure is mathematically unsupported by the given MTBF and MTTR values.
99.993% overstates the result; the correct calculation of 10,000 / 10,002 yields 99.98%, not 99.993%, indicating an arithmetic error.
99.2% would indicate the firewall fails the requirement, but this figure does not result from correctly applying MTBF / (MTBF + MTTR) to the provided values of 10,000 and 2.
Concept tested: Availability calculation using MTBF and MTTR
Source: https://csrc.nist.gov/glossary/term/mean_time_between_failures
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