These are the questions most candidates get wrong. They require connecting multiple concepts, reading tricky output, or knowing edge-case behaviour that isn't on most study cards. Practising them trains you to operate under uncertainty — a necessary skill on the real exam.
Start Scenario PracticeWhich three statements about using Python for device inventory and data serialization in network automation are true? (Choose three.)
Explanation: The correct answers describe practical uses of Python for inventory management and data handling. The incorrect options either misattribute YAML's features (it does not support comments natively in all parsers) or incorrectly state that JSON is always more human-readable than YAML (YAML is often considered more readable).
A network engineer runs the following command on Switch SW1: SW1# show vlan id 10 VLAN ID: 10 VLAN Name: Sales VLAN Type: Ethernet VLAN State: active MTU: 1500 Remote SPAN VLAN: No Primary VLAN ID: 10 Private VLAN Type: Primary Associated Secondary VLAN IDs: 100, 200 Based on this output, what can be concluded?
Explanation: The output shows VLAN 10 configured as a Primary VLAN with associated secondary VLANs 100 and 200, which is the defining characteristic of a primary private VLAN. This is confirmed by the fields 'Private VLAN Type: Primary' and 'Associated Secondary VLAN IDs: 100, 200'. Therefore, VLAN 10 is a primary private VLAN, not a normal data VLAN.
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?
Explanation: 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.
An engineer is deploying QoS on a WAN link between two sites using a Cisco ISR 4451 router. The link is a 10 Mbps MPLS circuit. The engineer wants to ensure that voice traffic (EF) is never dropped, even during congestion. The current policy uses a single class map for voice with a policer that drops excess traffic. During peak hours, users report choppy voice calls. What change should the engineer make?
Explanation: A policer drops traffic that exceeds the configured rate, which causes choppy voice calls during congestion. Changing the policer to a shaper allows the router to buffer excess voice traffic (EF) instead of dropping it, ensuring that voice packets are never dropped and are instead queued and transmitted when bandwidth becomes available. This aligns with the requirement that voice traffic (EF) is never dropped, even during congestion, as a shaper provides a smoothing effect without discarding packets.
A network engineer is troubleshooting an STP issue in a network that uses Rapid PVST+. The network has a root bridge (SW1) and a secondary root bridge (SW2). The engineer notices that after a link failure between SW1 and SW2, the network takes longer than expected to converge. The engineer checks the configuration and finds that SW2 has the 'spanning-tree uplinkfast' command enabled. The engineer also notices that SW2 has a lower priority than SW1. What is the most likely cause of the slow convergence?
Explanation: UplinkFast is a legacy STP feature that is incompatible with Rapid PVST+. When enabled on a switch running Rapid PVST+, it forces the switch to revert to 802.1D STP convergence behavior on the affected ports, disabling the rapid transition mechanisms (such as proposal/agreement and sync). This causes the network to take longer to converge after a link failure, as the switch falls back to the slower listening and learning states.
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Practice all Hard Difficulty QuestionsThese are the questions most candidates get wrong. They require connecting multiple concepts, reading tricky output, or knowing edge-case behaviour that isn't on most study cards. Practising them trains you to operate under uncertainty — a necessary skill on the real exam. These appear throughout the 350-401 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 350-401. Practicing each scenario type ensures you're ready for any format.
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