Will the backup keep the service available?
Verify that an independent path can carry the service within budget.
The question
You assess protection for a 6 Gb/s enterprise service in Australia. The customer wants service to continue if one duct failure takes out the primary path. The budget for one standby is AUD 9,000 per month. The network owner accepts route evidence and service tests.
| Usable Gb/s | Provider | |
|---|---|---|
| Primary A | 10 | Provider 1 |
| Proposed standby B | 8 | Provider 2 |
The supplier summaries show who provides each path. Physical separation still needs a check.
“What could B carry and what would you verify before offering protection?”
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A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: Check surviving capacity
What would you do here? Open the move you’d make.
Check the standby’s capacity on its own.
The strong move. The commitment depends on the capacity of the path that would remain available.
Add A and B and promise 18 Gb/s during a primary outage.
Tempting, but it misses something. A is unavailable in the scenario. Only the surviving path contributes capacity.
See how a strong candidate says it
“If B remains available during the defined failure, its 8 Gb/s could carry the 6 Gb/s service with 2 Gb/s spare.”
Why it works: The commitment depends on the capacity of the path that would remain available.
Step 2: Check physical separation
What would you do here? Open the move you’d make.
Keep protection conditional and ask the owner to verify the physical paths.
The strong move. Different suppliers can use a common duct. The records show where each path goes.
Assume the routes are independent because the providers differ.
Tempting, but it misses something. The supplier names give no evidence that the two paths use separate ducts.
See how a strong candidate says it
“I’d ask the network owner for the built route records and a check of shared segments. I’d leave protection conditional until those records are verified.”
Why it works: Different suppliers can use a common duct. The records show where each path goes.
Step 3: Use the verified routes
The owner confirms A and B share a duct. Both would be unavailable in the defined failure. The owner verifies separate paths from A for C and D. C offers 7 Gb/s at AUD 8,000 per month. D offers 5 Gb/s at AUD 4,000 per month.
What would you do here? Open the move you’d make.
Recommend C, subject to the network owner’s service test and approval.
The strong move. The records show C survives the defined failure. Its capacity and cost fit the request.
Choose D because its separate path costs less.
Tempting, but it misses something. D can carry 5 Gb/s. The service needs 6 Gb/s for the defined failure.
See how a strong candidate says it
“The shared duct leaves 0 Gb/s across A and B in this failure. C has 7 Gb/s and costs AUD 8,000 per month. That gives 1 Gb/s spare and leaves AUD 1,000 in budget. D is 1 Gb/s short. I’d ask the owner to test and approve C.”
Why it works: The records show C survives the defined failure. Its capacity and cost fit the request.
Step 4: State the protection limit
What would you do here? Open the move you’d make.
State the defined event and the remaining approval condition.
The strong move. The offer covers the agreed event and capacity. The owner still has to confirm the service test.
Guarantee uninterrupted service for every outage.
Tempting, but it misses something. The path evidence only covers the defined duct failure. Other failure conditions need their own checks.
See how a strong candidate says it
“I’d tell the customer C can support the 6 Gb/s requirement for the defined duct failure if the owner confirms the test. I’d record the failure scope and approval status with the offer.”
Why it works: The offer covers the agreed event and capacity. The owner still has to confirm the service test.
The answer, in short
- Decision
- Recommend C within the budget and ask the network owner to approve the protection test.
- Because
- C has 7 Gb/s for the 6 Gb/s requirement during the defined duct failure.
- What would change it
- A change in the physical path, available capacity or test result would reopen the offer.
Common traps
- Treating different suppliers as proof of physical separation.
- Comparing cost before checking surviving capacity.
Check your answer
- I used the capacity of the surviving path.
- I checked for shared segments.
- I compared the options with the capacity and budget.
- I gave the customer the tested scope and approval condition.
Last reviewed Nov 11, 2026