- A
The summary LSA is being suppressed because the ABR is also advertising a type 5 external route for the same prefix.
Why wrong: Type 5 LSAs are for external routes; they do not suppress type 3 summaries.
- B
The ABR has no interface in area 1 that matches the summary range; the 'area range' command requires a directly connected interface.
Why wrong: The range summarizes routes learned via intra-area LSAs, not necessarily a directly connected interface.
- C
The 'area range' command only works after a routing process restart; a 'clear ospf database' is required to regenerate LSAs.
Why wrong: OSPF updates are triggered by topology changes; a restart is not necessary.
- D
The individual prefixes from area 1 may not be installed in the inet.0 routing table on the ABR, possibly due to a missing OSPF adjacency or a route filter blocking them.
Without a valid inter-area route, the ABR cannot generate the summary.
Quick Answer
The answer is that the individual prefixes from area 1 are likely not installed in the ABR’s inet.0 routing table, possibly due to a missing OSPF adjacency or a route filter blocking them. This is correct because the OSPF area range summarization prerequisites require that the ABR first have an active inter-area route for each component prefix in its routing table before it can generate a Type 3 summary LSA; without those routes in inet.0, the `area 1 range` command produces no summary. On the JNCIA-Junos exam, this concept tests your understanding that OSPF summarization is a routing-table-driven process, not a configuration-only toggle—a common trap is assuming the range command alone creates the summary. Remember the memory tip: “No route in table, no summary label.”
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
A service provider is migrating from a legacy core network to Juniper MX series routers. In the new design, each MX router runs OSPF as the IGP with a single backbone area 0 and multiple non-backbone areas for customer aggregation. The network uses route summarization at area border routers (ABRs) to reduce routing table size. During testing, engineers notice that some customer prefixes from area 1 are not being propagated to the backbone area, even though the ABR has a valid route to those prefixes and has been configured with 'area 1 range 192.168.0.0/16'. The ABR is an MX240 running Junos 21.4R1. The 'show ospf route' command on the ABR shows the individual customer prefixes (e.g., 192.168.1.0/24) in the OSPF routing table, but the summary route is not present in the backbone area's database. Additionally, 'show ospf database summary' on a backbone router does not show the summary LSA for 192.168.0.0/16. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The individual prefixes from area 1 may not be installed in the inet.0 routing table on the ABR, possibly due to a missing OSPF adjacency or a route filter blocking them.
Option D is correct because the 'area range' command requires an active inter-area route to exist in the routing table; if the ABR does not have a route to the individual prefixes in the inet.0 table (perhaps due to filtering or no OSPF adjacency), the summary is not generated. Option A is incorrect; the area range command itself works, but a restart is not required. Option B is incorrect; the area range does not need a matching interface; it summarizes routes learned from the area. Option C is incorrect; the summary LSA is type 3, not type 5.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 summary LSA is being suppressed because the ABR is also advertising a type 5 external route for the same prefix.
Why it's wrong here
Type 5 LSAs are for external routes; they do not suppress type 3 summaries.
- ✗
The ABR has no interface in area 1 that matches the summary range; the 'area range' command requires a directly connected interface.
Why it's wrong here
The range summarizes routes learned via intra-area LSAs, not necessarily a directly connected interface.
- ✗
The 'area range' command only works after a routing process restart; a 'clear ospf database' is required to regenerate LSAs.
Why it's wrong here
OSPF updates are triggered by topology changes; a restart is not necessary.
- ✓
The individual prefixes from area 1 may not be installed in the inet.0 routing table on the ABR, possibly due to a missing OSPF adjacency or a route filter blocking them.
Why this is correct
Without a valid inter-area route, the ABR cannot generate the summary.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Routing Fundamentals — This question tests Routing Fundamentals — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The individual prefixes from area 1 may not be installed in the inet.0 routing table on the ABR, possibly due to a missing OSPF adjacency or a route filter blocking them. — Option D is correct because the 'area range' command requires an active inter-area route to exist in the routing table; if the ABR does not have a route to the individual prefixes in the inet.0 table (perhaps due to filtering or no OSPF adjacency), the summary is not generated. Option A is incorrect; the area range command itself works, but a restart is not required. Option B is incorrect; the area range does not need a matching interface; it summarizes routes learned from the area. Option C is incorrect; the summary LSA is type 3, not type 5.
What should I do if I get this JNCIA-JUNOS question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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