STP and Rapid PVST+ questions test your understanding of root bridge election, port roles, port states, and protection features. These appear in MCQ, multi-select, and drag-and-drop formats on the CCNA because STP behaviour is non-intuitive and frequently misunderstood.
Start Scenario PracticeA network administrator is troubleshooting a switch that is experiencing high CPU utilization. The administrator suspects a switching loop. Which command should be used to verify whether Spanning Tree Protocol has blocked any ports to prevent a loop?
Explanation: Spanning Tree Protocol prevents loops by blocking redundant ports. When a loop is suspected, checking STP status is essential. The show spanning-tree command provides detailed information about port roles and states, including which ports are blocking. If STP is functioning correctly, it will have blocked ports to break the loop, though high CPU may still occur during topology changes.
A network administrator is configuring a switch port that connects to a server. The administrator wants to ensure that the port does not participate in Spanning Tree Protocol (STP) and immediately transitions to the forwarding state when the link comes up. Which feature should be configured on this port?
Explanation: PortFast is the correct feature because it allows an access port to skip the STP listening and learning states and go straight to forwarding, which is ideal for server connections. BPDU Guard, Root Guard, and Loop Guard are STP enhancement features that protect the topology but do not provide immediate forwarding.
What is the primary function of the Spanning Tree Protocol (STP) in a network with redundant links?
Explanation: STP's primary function is to prevent Layer 2 loops by creating a loop-free logical topology. In a network with redundant links, STP elects a root bridge, determines root ports and designated ports, and blocks redundant ports to eliminate loops. This prevents broadcast storms and MAC address table instability that would otherwise occur due to frames circulating indefinitely.
A network technician is reviewing a Cisco switch configuration and sees that PortFast and BPDU Guard are enabled on an access port that connects to a user's desktop. The user calls and reports that they accidentally plugged a small unmanaged switch into that wall jack so they could connect both a PC and a printer. Shortly afterward, the port stopped forwarding traffic. What is the most likely explanation for this behavior?
Explanation: When PortFast is enabled on an access port, it allows the port to transition immediately to the forwarding state, bypassing the listening and learning states. To prevent loops, BPDU Guard is often enabled alongside PortFast. If a BPDU is received on a PortFast-enabled port, BPDU Guard err-disables the port. In this scenario, connecting an unmanaged switch causes BPDUs to be sent, triggering BPDU Guard and disabling the port.
Which mechanism does the Spanning Tree Protocol (STP) use to detect and prevent Layer 2 loops in a network with redundant links?
Explanation: STP prevents loops by designating certain redundant ports as 'blocked'. It uses Bridge Protocol Data Units (BPDUs) to communicate between switches, elect a Root Bridge, and calculate the shortest path to that root. By logically disabling redundant paths, STP ensures a loop-free topology while maintaining the ability to automatically restore connectivity if a primary link fails, providing essential resilience for switched LANs.
STP and Rapid PVST+ questions test your understanding of root bridge election, port roles, port states, and protection features. These appear in MCQ, multi-select, and drag-and-drop formats on the CCNA because STP behaviour is non-intuitive and frequently misunderstood. These appear throughout the 100-150 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 100-150. Practicing each scenario type ensures you're ready for any format.
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