This LFCS practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which THREE conditions can cause an 'RTNETLINK answers: File exists' error when adding a static route?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route exists with a different metric but same destination and netmask.
Option A is correct because the Linux kernel's routing table uses the destination and netmask as a unique key for route entries. When adding a route with 'ip route add', if a route already exists with the same destination and netmask but a different metric, the kernel returns 'RTNETLINK answers: File exists' because the metric is not part of the uniqueness check — only the destination prefix and netmask are considered. The metric is a per-route property that does not disambiguate routes in the FIB (Forwarding Information Base).
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The route exists with a different metric but same destination and netmask.
Why this is correct
Metric is not part of the uniqueness; still duplicates.
Related concept
Read the scenario before looking for a memorised answer.
✗
The gateway is unreachable.
Why it's wrong here
Would give 'Network is unreachable' error.
✗
The interface is down.
Why it's wrong here
Would give 'Cannot assign requested address' or similar.
✓
The route exists with a different gateway but same destination and netmask.
Why this is correct
Still a duplicate route entry if the destination/netmask is the same.
Related concept
Read the scenario before looking for a memorised answer.
✓
The route already exists with the same destination and netmask (exact duplicate).
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume the metric or gateway differentiates routes, but the Linux kernel treats the destination/netmask pair as the sole unique identifier for a route entry, so any duplicate prefix — even with different metrics or gateways — triggers the 'File exists' error.
Trap categories for this question
Similar concept trap
Would give 'Cannot assign requested address' or similar.
Detailed technical explanation
How to think about this question
The 'RTNETLINK answers: File exists' error originates from the netlink socket interface (rtnetlink(7)) when the kernel's fib_table_insert() function detects a duplicate prefix (destination/netmask) in the FIB. The kernel uses a longest-prefix-match lookup, and the route's metric (priority) is only used for multipath routing or policy routing (ip rule), not for uniqueness. In real-world scenarios, this error commonly occurs when a DHCP client or network manager has already installed a default route, and a manual 'ip route add default via ...' command is attempted without the 'replace' or 'append' keyword.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The route exists with a different metric but same destination and netmask. — Option A is correct because the Linux kernel's routing table uses the destination and netmask as a unique key for route entries. When adding a route with 'ip route add', if a route already exists with the same destination and netmask but a different metric, the kernel returns 'RTNETLINK answers: File exists' because the metric is not part of the uniqueness check — only the destination prefix and netmask are considered. The metric is a per-route property that does not disambiguate routes in the FIB (Forwarding Information Base).
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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