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This page has 5 Spanning Tree Protocol Scenarios scenario questions for the 100-150 exam, each with detailed explanations and wrong-answer analysis.

How should I approach 100-150 scenario questions?

Read the full scenario before looking at the answer options. Identify the constraint or requirement in the scenario, then eliminate options that are generally true but wrong for this specific case. Scenario questions reward careful reading over pattern matching.

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Scenario-based practice

Spanning Tree Protocol Scenarios

Practise 100-150 STP questions covering root bridge election, bridge IDs, port roles (root, designated, blocking), path cost, PortFast, BPDU Guard, and how spanning tree prevents Layer 2 loops.

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scenario questions

100-150

exam code

Cisco

vendor

Scenario guide

How to approach spanning tree protocol scenarios

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.

Quick answer

STP questions usually test root bridge election, port roles, path cost and how spanning tree prevents Layer 2 loops.

Root bridge election and bridge ID comparison.

Root ports, designated ports and blocked/alternate ports.

Path cost and how switches choose the best path to the root bridge.

PortFast, BPDU Guard and common Layer 2 loop-prevention behaviour.

Practice scenarios

Question 1mediummultiple choice

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A 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?

A
show interfaces status
Why it fails: show interfaces status displays speed, duplex, and VLAN assignment for each port, but it does not show Spanning Tree Protocol states. It can indicate connected or notconnected, but not whether STP has blocked a port. This command is useful for physical layer verification, not for detecting STP topology changes or loops.
B
show running-config
Why it fails: show running-config displays the current configuration, including STP settings, but it does not show operational STP states or port roles. It can confirm if STP is enabled, but not whether ports are currently blocking. For real-time STP status, operational commands like show spanning-tree are required.
C
show spanning-tree
show spanning-tree displays the STP status for each VLAN, including root bridge, port roles, and port states. If a loop exists, STP will place redundant ports in blocking state. This command reveals which ports are blocking and can confirm whether STP is actively preventing a loop, helping diagnose high CPU caused by topology changes.
D
show mac address-table
Why it fails: show mac address-table lists MAC addresses learned per port. A loop can cause MAC address flapping, where the same MAC appears on multiple ports, but this command alone does not show STP states. While useful for detecting flapping, it does not directly verify whether STP has blocked ports to prevent a loop.

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Question 2hardmultiple choice

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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?

A
Root Guard
Why it fails: Root Guard is used to prevent a port from becoming the root port or the switch from becoming the root bridge. It does not affect the STP state transition timing. When a superior BPDU is received, the port is placed in a root-inconsistent state, which blocks traffic. It is unrelated to immediate forwarding for end devices.
B
Loop Guard
Why it fails: Loop Guard prevents a port from erroneously entering the forwarding state if BPDUs stop being received, which could cause a loop. It does not expedite the STP convergence for access ports. It is typically used on ports connecting to other switches, not on server ports that should immediately forward traffic.
C
BPDU Guard
Why it fails: BPDU Guard is a safety feature that disables a port if it receives a BPDU, which helps prevent loops when PortFast is enabled. It does not make the port transition immediately to forwarding; instead, it err-disables the port upon receiving a BPDU. It is often used in conjunction with PortFast but does not provide the fast transition itself.
D
PortFast
PortFast is a Cisco feature that allows an access port to bypass the listening and learning STP states, immediately entering the forwarding state. It is designed for ports connected to end devices like servers or workstations, reducing the time they wait for network connectivity. However, it should not be used on ports connecting to switches to avoid loops.

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Question 3mediummultiple choice

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What is the primary function of the Spanning Tree Protocol (STP) in a network with redundant links?

A
To increase the total bandwidth of multiple links.
Why it fails: Link aggregation, such as EtherChannel, is used to increase bandwidth. STP specifically blocks redundant links to prevent loops, which effectively limits the use of those specific paths for data transmission. Therefore, STP does not increase bandwidth; it actually ensures a loop-free tree, which may ignore some paths.
B
To prevent Layer 2 loops.
STP maintains a loop-free network topology by dynamically disabling redundant paths. If a primary path fails, STP can re-enable a blocked path to restore connectivity. This functionality is essential for avoiding broadcast storms and MAC table corruption that occur when multiple redundant Layer 2 links exist.
C
To assign IP addresses to switches.
Why it fails: IP address management is handled by DHCP or static configuration. STP is entirely a Layer 2 protocol focused on path management and topology control. It has no role in the assignment or management of IP addresses, which are handled at the Network layer of the OSI model.
D
To route traffic between different VLANs.
Why it fails: Inter-VLAN routing is performed by Layer 3 devices like routers or Layer 3 switches. STP is strictly a Layer 2 protocol that focuses on maintaining a loop-free topology; it does not perform any routing tasks or communicate traffic between different broadcast domains or IP subnets at all.

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Question 4mediummultiple choice

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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?

A
The switch detected a MAC address flapping condition and applied a default port security violation.
Why it fails: MAC address flapping can occur when a loop exists, but port security is not enabled by default on Cisco switches. Without port security configured, the switch would not take action based solely on MAC flapping. The scenario does not mention port security, so this is not the cause.
B
The unmanaged switch sent BPDUs that were received on the PortFast port, and BPDU Guard placed the port into the err-disabled state.
BPDU Guard is designed to protect PortFast ports from receiving BPDUs. When the unmanaged switch is connected, it begins sending BPDUs, and BPDU Guard immediately err-disables the port to prevent a potential switching loop. This matches the scenario where the port stops forwarding traffic after the switch is attached.
C
The unmanaged switch negotiated a speed of 10 Mbps, which is unsupported on the access port.
Why it fails: Cisco access ports typically support multiple speeds, including 10 Mbps, depending on the hardware. A speed of 10 Mbps would not cause the port to stop forwarding traffic unless specifically configured otherwise. This is not a standard behavior and does not match the described symptom.
D
The unmanaged switch caused a duplex mismatch, and the port was automatically disabled by the switch fabric.
Why it fails: A duplex mismatch typically causes performance issues such as collisions and slow throughput, but it does not cause the port to be err-disabled by the switch. The switch would not automatically shut down the port solely due to a duplex mismatch. Therefore, this does not explain why the port stopped forwarding traffic.

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Question 5hardmultiple choice

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Which mechanism does the Spanning Tree Protocol (STP) use to detect and prevent Layer 2 loops in a network with redundant links?

A
Broadcast storm suppression
Why it fails: Broadcast storm suppression is a safety feature that limits the volume of broadcast traffic, but it is not the mechanism that prevents loops. STP actively blocks ports to stop loops from forming in the first place, whereas suppression merely mitigates the symptoms of an existing loop.
B
Exchange of Bridge Protocol Data Units (BPDUs)
Switches use BPDUs to communicate topology information and determine the spanning tree. By electing a root bridge and calculating port roles, STP identifies and disables redundant links that would otherwise cause infinite broadcast loops, ensuring the network remains stable and functional under all conditions.
C
Source MAC address learning
Why it fails: MAC address learning is the standard process for building a forwarding table. It does not prevent loops; in fact, redundant links without STP cause MAC table instability, leading to frame duplication and network saturation as the switch receives the same address from multiple ports simultaneously.
D
Packet TTL expiration
Why it fails: Time-to-Live (TTL) is a Layer 3 field used to prevent packets from looping indefinitely in a routed network. It has no effect on Layer 2 frames, which do not contain a TTL field, meaning Layer 2 loops require a protocol like STP for mitigation.

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These 100-150 practice questions are part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style 100-150 questions with detailed explanations, topic-based practice, mock exams, readiness tracking, and study analytics.

Spanning Tree Protocol Scenarios

How to approach spanning tree protocol scenarios

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Practice scenarios

A 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?

What is the primary function of the Spanning Tree Protocol (STP) in a network with redundant links?

Which mechanism does the Spanning Tree Protocol (STP) use to detect and prevent Layer 2 loops in a network with redundant links?