The answer is that a contributing route must exist for the aggregate route to become active. In Junos, an aggregate route is a summary address that remains hidden in the routing table until at least one more specific route—known as a contributing route—falls within its range. This requirement ensures the aggregate is only advertised when there is actual reachability to a subnet within the summarized block. On the JNCIA-Junos exam, this concept tests your understanding of how Junos handles route aggregation differently from a static summary; a common trap is assuming a static route to the aggregate itself will activate it, but the contributing route must be a more specific prefix. Remember the memory tip: “No specific, no summary”—without a contributing route, the aggregate stays inactive.
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.
You configure an aggregate route for 172.16.0.0/16. What must exist for this aggregate to be active in the routing table?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A contributing route (more specific route within the aggregate range)
The correct answer is A. An aggregate route requires at least one contributing route (a more specific route within the aggregate's range) to become active. Without a contributing route, the aggregate remains inactive. Option B is not required, though a static route can serve as contributor. Option C is not required. Option D is false; a contributing route must exist.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
A directly connected interface on that subnet
Why it's wrong here
Direct routes are not required; any route within the aggregate range works.
✗
A static route to the aggregate prefix
Why it's wrong here
The aggregate itself is a static-like route; it does not need another static route. It needs a contributor.
✓
A contributing route (more specific route within the aggregate range)
Why this is correct
Aggregates are active only when a more specific route exists in the routing table.
Nothing; the aggregate is always active once committed.
Why it's wrong here
Aggregates are inactive if no contributor exists.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related JNCIA-JUNOS subnetting questions on CIDR, address ranges, and subnet selection.
Routing Fundamentals — This question tests Routing Fundamentals — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: A contributing route (more specific route within the aggregate range) — The correct answer is A. An aggregate route requires at least one contributing route (a more specific route within the aggregate's range) to become active. Without a contributing route, the aggregate remains inactive. Option B is not required, though a static route can serve as contributor. Option C is not required. Option D is false; a contributing route must exist.
What should I do if I get this JNCIA-JUNOS question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related JNCIA-JUNOS subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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