The correct answer is the static route to 172.16.0.0/16 because the router applies the longest prefix match rule, selecting the most specific route that matches the destination address 172.16.1.100. Among the three static routes, the default route 0.0.0.0/0 matches any destination but has the least specificity, while the route to 10.0.0.0/8 does not match the destination at all. The route to 172.16.0.0/16, with a prefix length of 16, is the most specific match, so it is chosen over the less specific default route. On the JNCIA-Junos exam, this concept tests your understanding of how Junos evaluates static routes and forwarding decisions, often appearing in scenario-based questions where a longer prefix overrides a shorter one, even if the shorter route has a better administrative distance. A common trap is assuming a default route or a route with a lower metric will win, but longest prefix match always takes priority first. Remember: the longer the mask, the stronger the match.
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
```
user@router> show route protocol static
inet.0: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
0.0.0.0/0 *[Static/5] 00:10:00, metric2 0
> to 192.168.1.1 via ge-0/0/0.0
10.0.0.0/8 *[Static/5] 00:05:00, metric2 0
> to 10.0.0.1 via ge-0/0/1.0
172.16.0.0/16 *[Static/5] 00:02:00, metric2 0
> to 172.16.0.1 via ge-0/0/2.0
```
Refer to the exhibit. The router has three static routes configured. Which route will be used to forward a packet destined to 172.16.1.100?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The static route to 172.16.0.0/16
The router will use the most specific matching route for the destination 172.16.1.100. The static route to 172.16.0.0/16 has a prefix length of 16, which is more specific than the default route (0.0.0.0/0) and the route to 10.0.0.0/8 (which does not match the destination). Therefore, the route to 172.16.0.0/16 is selected based on the longest-prefix-match rule.
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 default route (0.0.0.0/0)
Why it's wrong here
The default route is used only when no more specific route matches.
✗
The router will drop the packet.
Why it's wrong here
A matching route exists, so the packet will be forwarded.
✗
The static route to 10.0.0.0/8
Why it's wrong here
The destination does not match the 10.0.0.0/8 prefix.
✓
The static route to 172.16.0.0/16
Why this is correct
The destination 172.16.1.100 is within the 172.16.0.0/16 network.
Related concept
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 a default route will always be used as a catch-all, but they forget that a more specific static route (like 172.16.0.0/16) will take precedence over the default route for matching destinations.
Detailed technical explanation
How to think about this question
Junos uses a longest-prefix-match algorithm in its forwarding table (FIB) to select the best route for packet forwarding. Even if multiple static routes exist, the route with the longest subnet mask (highest prefix length) that matches the destination IP is chosen, regardless of administrative distance or metric. In this scenario, the /16 route is more specific than the /0 default, and the /8 route does not match at all, so the /16 route wins.
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.
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: The static route to 172.16.0.0/16 — The router will use the most specific matching route for the destination 172.16.1.100. The static route to 172.16.0.0/16 has a prefix length of 16, which is more specific than the default route (0.0.0.0/0) and the route to 10.0.0.0/8 (which does not match the destination). Therefore, the route to 172.16.0.0/16 is selected based on the longest-prefix-match rule.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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