CAS-002 · Question #563
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%. Hardware availability is calculated as MTBF divided by MTBF plus MTTR, and the firewall's resulting 99.98% availability 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
(19 responses)- A68% (13)
- B5% (1)
- C5% (1)
- D21% (4)
Why each option
Hardware availability is calculated as MTBF divided by MTBF plus MTTR, and the firewall's resulting 99.98% availability exceeds the 99.9% requirement.
Applying the standard availability formula: 10,000 / (10,000 + 2) = 10,000 / 10,002 = 0.9998, or 99.98%. Since 99.98% exceeds the required 99.9% service availability, the firewall meets the stated target, making this the most accurate statement.
85% availability would imply approximately 1,314 hours of annual downtime, which is mathematically inconsistent with an MTBF of 10,000 hours and an MTTR of only 2 hours.
99.993% is an incorrect result; the correct application of MTBF/(MTBF+MTTR) with these values yields 99.98%, not 99.993%.
99.2% availability implies roughly 70 hours of annual downtime, which cannot be derived from the given MTBF and MTTR values using the standard formula.
Concept tested: Availability calculation using MTBF and MTTR
Source: https://csrc.nist.gov/glossary/term/availability
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