The correct answer is to set the OSPF metric on the backup interface to a higher value. This works because OSPF uses the Shortest Path First algorithm to select routes based on the lowest cumulative metric; by raising the metric on one path, you break the equal-cost multipath condition, forcing all traffic for the prefix to use the lower-metric primary path while retaining automatic failover if that path fails. On the JNCIA-Junos exam, this scenario tests your understanding of OSPF metric manipulation and ECMP behavior—a common trap is assuming you must disable load balancing globally or use policy-options, but the simplest Junos-native method is adjusting the metric at the interface level. Remember the key principle: OSPF always prefers the lowest metric, so to stop ECMP, you just make one path costlier. A useful memory tip is “metric up, path down”—raising the metric drops that route from active use.
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An engineer observes that traffic destined to 203.0.113.0/24 is being load-balanced across two equal-cost paths via OSPF. The engineer wants to ensure that all traffic for this prefix uses only one path unless the primary path fails. Which configuration change should be made?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set the OSPF metric on the backup interface to a higher value.
Option B is correct because increasing the OSPFmetric on the backup interface makes that path less preferred in the SPF calculation, breaking the equal-cost multipath (ECMP) condition. OSPF selects routes based on the lowest metric; by raising the metric on one interface, the engineer ensures only the lower-metric path is used for forwarding, with automatic failover to the backup path if the primary fails.
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.
✗
Configure a static route for 203.0.113.0/24 with a lower preference.
Why it's wrong here
A static route would override OSPF and could cause loss of dynamic failover.
✓
Set the OSPF metric on the backup interface to a higher value.
Why this is correct
Increasing metric on one interface makes it less preferred, so OSPF will use the lower metric path as primary.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use policy-options to set a higher preference for one of the paths.
Why it's wrong here
OSPF route preference is per protocol, not per path; you can influence metric but not preference per path easily.
✗
Modify the load-balance configuration under forwarding-options.
Why it's wrong here
Load-balance configuration affects how traffic is distributed but does not create active/backup behavior.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates mistakenly think modifying load-balancing or preference settings can break ECMP, but OSPF ECMP is solely determined by equal metric values; only changing the metric (or using a feature like per-packet load balancing) directly controls path selection.
Detailed technical explanation
How to think about this question
OSPF uses the SPF algorithm to compute the shortest path based on cumulative cost (metric) per interface, where default cost is derived from interface bandwidth (reference bandwidth / interface bandwidth). To create a non-equal-cost path, the engineer must adjust the metric on one interface (e.g., `set protocols ospf area 0 interface ge-0/0/1 metric 100`), which ensures the SPF tree selects only the lower-metric path; the higher-metric path remains in the routing table as a backup only if it is the next best alternative. In real-world scenarios, this technique is often used for traffic engineering, such as directing traffic over a higher-bandwidth link while keeping a lower-bandwidth link as a failover.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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.
Routing Fundamentals — This question tests Routing Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Set the OSPF metric on the backup interface to a higher value. — Option B is correct because increasing the OSPF metric on the backup interface makes that path less preferred in the SPF calculation, breaking the equal-cost multipath (ECMP) condition. OSPF selects routes based on the lowest metric; by raising the metric on one interface, the engineer ensures only the lower-metric path is used for forwarding, with automatic failover to the backup path if the primary fails.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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