JN0-106 Routing Fundamentals Practice Question
Which three characteristics are true for OSPF? (Choose three.)
⚠ Common exam trap
A common mix-up: candidates 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.
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 VLSM (Variable Length Subnet Masks), so option B is correct because it carries subnet mask information in its LSAs, allowing classless, hierarchical addressing. Option C is correct because OSPF's default metric is cost, which is derived from interface bandwidth (reference bandwidth divided by interface bandwidth). Option D is correct because OSPF uses the Dijkstra Shortest Path First (SPF) algorithm to compute the loop-free shortest-path tree from the link-state database. Option A is incorrect because the Bellman-Ford algorithm is used by distance-vector protocols such as RIP, not OSPF. Option E is incorrect because OSPF is classless and explicitly supports VLSM, contradicting the statement.
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
OSPF is a link-state protocol, not distance-vector. Distance-vector protocols like RIP use Bellman-Ford to compute routes based on hop counts, distributing entire routing tables to neighbors. OSPF instead floods link-state advertisements to build a complete topological map and then runs Dijkstra's SPF algorithm locally. Therefore, attributing Bellman-Ford to OSPF confuses the fundamental algorithm used by link-state versus distance-vector protocols.
- ✓
Supports VLSM (Variable Length Subnet Masks)
Why this is correct
OSPF is a classless routing protocol, meaning it carries the subnet mask in its routing updates, enabling VLSM and CIDR. This allows different interfaces on the same network to use different-length subnet masks, optimizing address space utilization. In contrast, classful protocols like RIPv1 cannot advertise subnet masks and thus cannot support VLSM, whereas OSPF's type-1 and type-3 LSAs explicitly include the mask.
- ✓
Uses cost as the metric
Why this is correct
OSPF's metric is called cost, which is inversely proportional to interface bandwidth: cost = reference bandwidth / interface bandwidth (default reference is typically 100 Mbps). The cost is the cumulative sum of outgoing interface costs along the path, and lower cost paths are preferred. This is a more sophisticated metric than hop count, ensuring OSPF chooses higher-bandwidth paths over lower-bandwidth ones.
- ✓
Uses the SPF (Shortest Path First) algorithm
Why this is correct
OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree from the router to all destinations based on the link-state database. Each router independently runs SPF after converging its LSDB, ensuring loop-free paths. This is fundamentally different from distance-vector's Bellman-Ford and allows OSPF to converge quickly and scale to larger networks.
- ✗
Is classful and does not support VLSM
Why it's wrong here
OSPF is inherently classless; it supports VLSM and CIDR by including subnet masks in LSAs. A classful protocol, such as RIPv1 or IGRP, forces all subnets of a major network to use the same mask and cannot carry mask information. OSPF's design from its inception included classless operation, making this statement false.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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