The answer is 50. The OSPF cost of an interface is explicitly set using the `ip ospf cost` command, which acts as a direct override to the default cost calculation derived from the reference bandwidth divided by interface bandwidth. In this configuration, the `ip ospf cost 50` command under GigabitEthernet0/0 takes absolute precedence, meaning the router will use 50 as the cost regardless of the physical link speed or any `auto-cost reference-bandwidth` settings. On the ENCOR 350-401 exam, this question tests your understanding that explicit configuration always wins over dynamic calculation—a common trap is assuming the cost is automatically computed from the interface bandwidth (e.g., 1 for GigabitEthernet), but the `ip ospf cost` command overrides that entirely. A useful memory tip is “explicit beats implicit”: if you see `ip ospf cost` configured, that number is the final cost, no math required.
CCNP OSPF Practice Question
This 350-401 practice question tests your understanding of ospf. 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.
Examine this OSPF configuration on router R4:
interface GigabitEthernet0/0
ip address 172.16.1.4 255.255.255.0
ip ospf 1 area 0
ip ospf cost 50
!
router ospf 1
router-id 4.4.4.4
network 172.16.0.0 0.0.255.255 area 0
What is the OSPF cost of the GigabitEthernet0/0 interface?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
50
The OSPF cost of an interface is determined by the `ip ospf cost` command, which explicitly overrides the default cost calculation based on reference bandwidth. Since R4's GigabitEthernet0/0 interface has `ip ospf cost 50` configured under it, the cost is set to 50 regardless of the interface bandwidth or reference bandwidth settings.
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.
✓
50
Why this is correct
The cost is explicitly set to 50 with the 'ip ospf cost' command.
Related concept
Read the scenario before looking for a memorised answer.
✗
1 (default for GigabitEthernet)
Why it's wrong here
The default cost is overridden by the explicit 'ip ospf cost' command.
✗
100 (derived from reference bandwidth 100 Mbps / 1 Gbps)
Why it's wrong here
The reference bandwidth is 100 Mbps by default, but the explicit cost command takes precedence.
✗
10 (derived from reference bandwidth 1000 Mbps / 100 Mbps)
Why it's wrong here
This calculation is incorrect; the explicit cost command overrides any derived value.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the precedence of the `ip ospf cost` command over the default cost calculation, trapping candidates who assume the default cost for GigabitEthernet (1) or who incorrectly calculate the cost using the reference bandwidth formula without considering the explicit configuration.
Trap categories for this question
Command / output trap
The default cost is overridden by the explicit 'ip ospf cost' command.
Detailed technical explanation
How to think about this question
OSPF cost is a metric used in the SPF algorithm to determine the best path; it is typically derived from the formula cost = reference bandwidth / interface bandwidth, where the default reference bandwidth is 100 Mbps (RFC 2328 suggests a default of 100 Mbps, but Cisco uses 100 Mbps by default). The `ip ospf cost` command allows manual override of this calculation, which is critical in multi-vendor environments or when using non-standard interface speeds to ensure consistent path selection. In real-world scenarios, network engineers often set costs manually to influence traffic engineering without changing physical link speeds.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 50 — The OSPF cost of an interface is determined by the `ip ospf cost` command, which explicitly overrides the default cost calculation based on reference bandwidth. Since R4's GigabitEthernet0/0 interface has `ip ospf cost 50` configured under it, the cost is set to 50 regardless of the interface bandwidth or reference bandwidth settings.
What should I do if I get this 350-401 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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