- A
Uses the Bellman-Ford algorithm
Why wrong: Bellman-Ford is used by distance-vector protocols like RIP.
- B
Supports VLSM (Variable Length Subnet Masks)
OSPF is a classless protocol and supports VLSM.
- C
Uses cost as the metric
OSPF uses cost based on interface bandwidth as its metric.
- D
Uses the SPF (Shortest Path First) algorithm
OSPF uses Dijkstra's SPF algorithm to compute shortest paths.
- E
Is classful and does not support VLSM
Why wrong: OSPF is classless; it supports VLSM and CIDR.
Question 358 of 514
OSPF Characteristics: SPF, Cost, and VLSM
This JNCIA-JUNOS practice question tests your understanding of routing fundamentals. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.)
Quick Answer
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.
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
✓
Supports VLSM (Variable Length Subnet Masks)
OSPF is a link-state routing protocol that supports Variable Length Subnet Masks (VLSM), which allows it to advertise subnets of different sizes within the same major network. This is a key advantage over classful protocols like RIPv1, as OSPF includes the subnet mask in its Link-State Advertisements (LSAs), enabling efficient IP address utilization.
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.
- ✗
Uses the Bellman-Ford algorithm
- ✓
Supports VLSM (Variable Length Subnet Masks)
- ✓
Uses cost as the metric
- ✓
Uses the SPF (Shortest Path First) algorithm
Why this is correct
OSPF uses Dijkstra's SPF algorithm to compute shortest paths.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Is classful and does not support VLSM
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse OSPF's metric (cost) with hop count or bandwidth, or mistakenly think OSPF is classful like RIPv1, but OSPF is inherently classless and uses a link-state algorithm, not Bellman-Ford.
Detailed technical explanation
How to think about this question
OSPF's cost metric is typically derived from the reference bandwidth (default 100 Mbps on Junos) divided by the interface bandwidth, allowing for fine-grained path selection. The SPF algorithm builds a complete topology map (Link-State Database) and computes the shortest path tree from the router's perspective, ensuring loop-free routing. In real-world deployments, OSPF's support for VLSM is critical for route summarization and efficient address space management in large enterprise networks.
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
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: Supports VLSM (Variable Length Subnet Masks) — OSPF is a link-state routing protocol that supports Variable Length Subnet Masks (VLSM), which allows it to advertise subnets of different sizes within the same major network. This is a key advantage over classful protocols like RIPv1, as OSPF includes the subnet mask in its Link-State Advertisements (LSAs), enabling efficient IP address utilization.
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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Last reviewed: Jul 4, 2026
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