The answer is to reduce the OSPF dead timer to 1 second and hello timer to 333 milliseconds. This directly addresses the core issue of OSPF convergence improvement by slashing the failure detection time from the default 40 seconds on a broadcast network down to roughly one second, which is the single most impactful change for the 30-second outage described. On the ENCOR 350-401 exam, this scenario tests your understanding that OSPF convergence is dominated by the dead timer, not just SPF calculation speed; a common trap is to choose Fast Hello (option A), which uses a sub-second hello but still relies on a multiplier for the dead interval, or iSPF (option D), which only helps after the neighbor is declared down. Remember the memory tip: "Dead first, then SPF" — you cannot converge on a failure you haven't detected yet, so shrinking the dead timer is always the fastest path to sub-5-second convergence without hardware upgrades.
350-401 Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. 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.
A large enterprise has a campus network with a collapsed core design. The core switch connects to two distribution switches, each serving several access switches. The network uses OSPF as the IGP. Recently, after a link failure between the core and distribution switch A, the network experienced a 30-second outage before converging. The engineer wants to improve convergence time to under 5 seconds. The budget is limited, so hardware upgrades are not an option. The engineer is considering the following actions:
A. Enable OSPF Fast Hello on all interfaces.
B. Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.
C. Implement OSPF LSA throttling with a minimum interval of 0 ms.
D. Use OSPF incremental SPF (iSPF).
Which action will provide the most significant improvement in convergence time for this scenario?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Why wrong: Fast Hello alone without reducing the dead timer may not reduce detection time significantly; it still waits for the dead timer unless the dead timer is also reduced.
B
Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.
This directly reduces failure detection to about 1 second, which is the main contributor to the 30-second outage.
C
Implement OSPF LSA throttling with a minimum interval of 0 ms.
Why wrong: LSA throttling speeds up propagation of LSAs after a change, but it does not reduce failure detection time.
D
Use OSPF incremental SPF (iSPF).
Why wrong: iSPF reduces SPF computation time for partial changes, but failure detection is still the bottleneck.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.
Option B is correct because reducing the OSPF dead timer to 1 second and hello timer to 333 milliseconds directly addresses the 30-second outage caused by the link failure. The default dead timer (40 seconds on broadcast networks) is the primary contributor to convergence delay, as OSPF must wait for the dead interval to expire before declaring a neighbor down. By lowering these timers, failure detection drops from 40 seconds to approximately 1 second, which is the most impactful single change for convergence under budget constraints.
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.
✗
Enable OSPF Fast Hello on all interfaces.
Why it's wrong here
Fast Hello alone without reducing the dead timer may not reduce detection time significantly; it still waits for the dead timer unless the dead timer is also reduced.
✓
Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.
Why this is correct
This directly reduces failure detection to about 1 second, which is the main contributor to the 30-second outage.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement OSPF LSA throttling with a minimum interval of 0 ms.
Why it's wrong here
LSA throttling speeds up propagation of LSAs after a change, but it does not reduce failure detection time.
✗
Use OSPF incremental SPF (iSPF).
Why it's wrong here
iSPF reduces SPF computation time for partial changes, but failure detection is still the bottleneck.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that Fast Hello (Option A) is the best way to speed convergence, but the trap is that Fast Hello alone does not reduce the dead timer below 1 second unless explicitly configured with a multiplier, and the dead timer is the dominant factor in failure detection time.
Detailed technical explanation
How to think about this question
OSPF dead timer default is 40 seconds on broadcast and NBMA networks (4x hello interval of 10 seconds). By setting the dead timer to 1 second and hello to 333 ms, the router detects a failure in roughly 1 second (plus propagation delay). This is a classic 'tuning OSPF timers' approach used in campus networks where fast convergence is critical, but it increases control plane overhead (hellos every 333 ms) and requires careful CPU planning. Cisco's implementation allows dead intervals as low as 1 second, but values below 1 second require the 'ip ospf dead-interval minimal' command (Fast Hello), which uses a different mechanism.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds. — Option B is correct because reducing the OSPF dead timer to 1 second and hello timer to 333 milliseconds directly addresses the 30-second outage caused by the link failure. The default dead timer (40 seconds on broadcast networks) is the primary contributor to convergence delay, as OSPF must wait for the dead interval to expire before declaring a neighbor down. By lowering these timers, failure detection drops from 40 seconds to approximately 1 second, which is the most impactful single change for convergence under budget constraints.
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.
Are there clue words in this question I should notice?
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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