The correct answer is that OSPF uses the SPF (Shortest Path First) algorithm, supports VLSM/CIDR, and employs cost as its metric. These three characteristics are fundamental because OSPF is a link-state protocol that relies on the Dijkstra algorithm to compute the shortest path tree, preventing loops and enabling fast convergence. Unlike distance-vector protocols, OSPF is classless by design, allowing variable-length subnet masking (VLSM) and CIDR for efficient IP address allocation, while its metric—cost—is typically derived from link bandwidth, not hop count. On the JNCIA-Junos exam, this question tests your ability to distinguish OSPF from RIP (which uses Bellman-Ford and is classful) and to recognize that OSPF’s SPF engine is its core differentiator. A common trap is confusing the metric with hop count or assuming OSPF is classful; remember that OSPF’s “S” in SPF stands for Shortest, not Simple. Memory tip: “OSPF Costs SPF for VLSM” — Cost, SPF, and VLSM are the three true characteristics.
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. 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 characteristics are true for OSPF? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Supports VLSM (Variable Length Subnet Masks)
The correct answers are A, C, and E. OSPF uses the Dijkstra SPF algorithm, supports VLSM/CIDR, and uses cost as its metric. It does not use Bellman-Ford (that's RIP). It is classless, not classful. So A, C, E are correct.
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.
✗
Uses the Bellman-Ford algorithm
Why it's wrong here
Bellman-Ford is used by distance-vector protocols like RIP.
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.
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: Supports VLSM (Variable Length Subnet Masks) — The correct answers are A, C, and E. OSPF uses the Dijkstra SPF algorithm, supports VLSM/CIDR, and uses cost as its metric. It does not use Bellman-Ford (that's RIP). It is classless, not classful. So A, C, E are correct.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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