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350-401 Architecture Practice Question

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?

⚠ Common exam trap

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.

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

✓

Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.

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.

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 changes only the hello transmission interval, not the dead timer itself. In Cisco OSPF, the dead interval remains at its configured value (or defaults to multiple seconds), so a router still waits the full dead period before declaring a neighbor down. Therefore, while Fast Hello increases hello frequency, failure detection remains the bottleneck, and the outage is not significantly reduced—both hello and dead timers must be lowered together to achieve sub-second convergence.

  • ✓

    Reduce OSPF dead timer to 1 second and hello timer to 333 milliseconds.

    Why this is correct

    The OSPF dead timer is the primary factor in convergence delay because it dictates how long a router waits for a missing hello before marking the neighbor unavailable. Setting the dead timer to 1 second ensures that a link failure is detected within roughly one second, and a hello interval of 333 milliseconds satisfies the standard three-hello requirement while maintaining a stable neighbor state. This direct reduction of the detection timer cuts the outage from tens of seconds to about one second, whereas other mechanisms only optimize post-detection processing.

  • ✗

    Implement OSPF LSA throttling with a minimum interval of 0 ms.

    Why it's wrong here

    OSPF LSA throttling controls the rate at which link-state advertisements are generated and flooded after a change is recognized, with a minimum interval of 0 ms merely eliminating the hold-down delay. However, this feature takes effect only after the router has already detected the topology change; it does nothing to accelerate the hello/dead timer process that identifies the failure. The 30-second outage is dominated by failure detection, so even instant LSA propagation leaves the detection wait untouched, and aggressive throttling can actually cause unnecessary CPU load.

  • ✗

    Use OSPF incremental SPF (iSPF).

    Why it's wrong here

    Incremental SPF (iSPF) is a post-detection optimization that reduces the CPU work during the Dijkstra calculation by recalculating only the affected branch of the shortest-path tree. It does not shorten the time needed to discover that a neighbor has failed; the router still must wait for the OSPF dead timer to expire before triggering any SPF computation. Since the dead timer is the bottleneck in a 30-second outage, iSPF offers no improvement to the detection phase and is therefore not a solution for this problem.

Visual reference

R1 R2 R3 R4 10 100 10 100 OSPF picks R1→R2→R4 (cost 20) over R1→R3→R4 (cost 200)

Quick reference

Routing Protocol Comparison

ProtocolMetricMax HopsAlgorithmType
RIP v2Hop count15Bellman-FordDistance vector
OSPFCost (bandwidth)UnlimitedDijkstra (SPF)Link state
EIGRPComposite metricUnlimitedDUALHybrid
IS-ISCostUnlimitedDijkstraLink state
BGPPolicy / attributesUnlimitedPath vectorPath 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

This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.