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 connectivity issues in a switched network. Users on VLAN 10 report intermittent connectivity to the server farm. The network uses Rapid PVST+ as the spanning-tree protocol. The administrator examines the switch that is the root bridge for VLAN 10 and notices that one of the uplink interfaces to an access switch is in a blocking state. What is the most likely cause of this issue?
Explanation: In Rapid PVST+, the root bridge for a VLAN should have all its ports in a forwarding state. If an uplink interface on the root bridge is blocking, it indicates that another switch is being elected as the root bridge for VLAN 10, likely because it has a lower spanning-tree priority. By checking and adjusting the priority on other switches, the administrator can ensure the intended switch becomes the root bridge, resolving the intermittent connectivity caused by suboptimal path selection.
A network administrator is troubleshooting connectivity loss in a switched network. All switches run Rapid PVST+. A host connected to an access port on SwitchC can no longer reach the default gateway. The access port is configured with PortFast and BPDU Guard. The administrator checks the interface status and finds it in an err-disabled state. What is the most likely cause of this issue?
Explanation: B is correct because BPDU Guard is designed to protect the spanning-tree topology by disabling a PortFast-enabled port if it receives a BPDU, placing the port in err-disabled state. Option A is incorrect: a root bridge election failure would not cause a port to err-disable; loops do not directly trigger this state without BPDU Guard. Option C is incorrect because PortFast and BPDU Guard work with all spanning-tree variants including Rapid PVST+. Option D is incorrect: a trunk misconfiguration alone would not cause err-disable unless BPDU Guard detects a BPDU on a PortFast port.
A Layer 2 switch port connected to an end host should move to forwarding quickly but also shut down if a BPDU is received. Which pair of features best supports that design?
Explanation: PortFast and BPDU Guard are the right pair. In plain language, PortFast makes an edge port usable quickly for a real end device, while BPDU Guard protects that same port by shutting it down if spanning-tree control traffic appears unexpectedly. This is a classic access-layer design. PortFast improves usability, and BPDU Guard improves safety. The best answer combines both functions.
You are connected to SW1 via the console. The network uses Rapid-PVST+ and you need to ensure that SW1 becomes the root bridge for VLAN 10 and VLAN 20. Additionally, configure PortFast and BPDU Guard on interface GigabitEthernet0/1, which connects to a workstation. After configuration, the workstation is moved and the port goes err-disabled. Diagnose the cause and recover the port without reloading the switch.
Explanation: SW1 is currently the root for VLAN 10 but not for VLAN 20. To become root for both VLANs, set the spanning-tree priority to a lower value (e.g., 4096) for each VLAN. The port Gi0/1 went err-disabled because it received a BPDU, which is unexpected on a PortFast edge port with BPDU Guard enabled. To recover, first identify and remove the BPDU source (likely another switch connected to that port), then use 'shutdown' followed by 'no shutdown' on the interface to bring it back up.
You are connected to R1, a multilayer switch acting as the STP root bridge. Configure Root Guard on the designated port toward R2 (G0/1), Loop Guard on the uplink port G0/2, and BPDU Guard on PortFast-enabled access port G0/3. After configuration, a superior BPDU is received on G0/1, causing it to be blocked by Root Guard; later, an unauthorized BPDU on G0/3 triggers err-disable. Troubleshoot and verify the expected port states.
Explanation: The root guard on G0/1 correctly blocked the port when a superior BPDU was received, preventing an unauthorized root bridge. Loop Guard was applied specifically to the uplink port G0/2 to prevent forwarding loops in case of uni-directional link failure. BPDU Guard on G0/3 placed the port into err-disable state upon receiving an unexpected BPDU, which protects the PortFast edge port. To restore G0/3, you must manually shut/no shut the interface after removing the offending device.
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Practice all Spanning Tree Protocol ScenariosSTP 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 200-301 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 200-301. Practicing each scenario type ensures you're ready for any format.
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