STP Spanning Tree Protocol Certification Exam — Questions and Answers

Question 1: What is a BPDU guard?

Firewall protection
A feature that disables a port if it receives a BPDU, protecting against unauthorized switches (Correct answer)
Password protection for BPDUs
A physical lock on a switch port

Correct answer: A feature that disables a port if it receives a BPDU, protecting against unauthorized switches

BPDU guard protects the network by shutting down ports that should not be receiving BPDUs, such as ports connected to end devices.

Question 2: What is the primary purpose of the Spanning Tree Protocol (STP)?

To provide IP address allocation
To prevent loops in a network topology (Correct answer)
To prevent broadcast storms
To increase network bandwidth

Correct answer: To prevent loops in a network topology

STP's primary purpose is to prevent switching loops in Ethernet networks, which can occur when redundant paths exist between network devices. Without STP, these loops would cause broadcast storms, MAC address table instability, and network collapse. STP achieves this by logically blocking redundant paths, ensuring only one active path exists between any two network segments.

Question 3: When implementing Rapid STP (802.1w) in a network design, which port role replaces the classic STP Blocking port for faster failover?

Designated port
Non-designated port
Alternate port (Correct answer)
Backup port

Correct answer: Alternate port

In RSTP, the Alternate port provides a pre-calculated backup path to the root and can transition to Forwarding immediately when the Root port fails.

Question 4: On each network segment, which STP port role is responsible for forwarding traffic toward the root bridge on behalf of that segment?

Root Port
Designated Port (Correct answer)
Backup Port
Non-Designated Port

Correct answer: Designated Port

The Designated Port on each segment is the one elected to forward traffic from that segment toward the root bridge.

Question 5: Which of the following correctly describes the 'spanning-tree pathcost method long' command?

Applies only to VLAN 1 path cost calculations
Enables RSTP long-path cost on a single interface
Switches the switch from 16-bit to 32-bit path cost values globally (Correct answer)
Forces all ports to use cost 200000 for path selection

Correct answer: Switches the switch from 16-bit to 32-bit path cost values globally

The 'spanning-tree pathcost method long' global command changes path cost calculation from the short (16-bit) method to the long (32-bit) method for better differentiation of high-bandwidth links.

Question 6: A port that is administratively shut down in STP is in which port state?

Listening
Disabled (Correct answer)
Discarding
Blocking

Correct answer: Disabled

The Disabled state applies to ports that have been administratively shut down and do not participate in STP at all.

Question 7: How do you change the STP root bridge priority?

Change the MAC address
Configure a lower bridge priority value on the desired root bridge switch (Correct answer)
Move the physical switch
Update the IP address

Correct answer: Configure a lower bridge priority value on the desired root bridge switch

Setting a lower priority value (e.g., 4096) ensures the desired switch wins the root bridge election.

Question 8: What is the STP max age timer?

The maximum time a switch stores BPDU information before discarding it, default 20 seconds (Correct answer)
The maximum age of a switch
The maximum number of VLANs
The maximum uptime allowed

Correct answer: The maximum time a switch stores BPDU information before discarding it, default 20 seconds

If a switch does not receive a BPDU within the max age period, it assumes a topology change and begins recalculating.

Question 9: A switched network uses PVST+ with the access layer running 802.1D timers. An engineer wants to reduce convergence time without migrating to RSTP. Which adjustment has the greatest impact?

Enable PortFast on trunk ports
Reduce the Forward Delay timer on the root bridge (Correct answer)
Increase the Max Age timer to 40 seconds
Increase the Hello timer on all access switches

Correct answer: Reduce the Forward Delay timer on the root bridge

Reducing the Forward Delay timer on the root bridge decreases the time ports spend in Listening and Learning states, directly cutting STP convergence time.

Question 10: What part of the brain controls balance and coordination?

Hypothalamus
Brainstem (Correct answer)
Cerebellum
Cerebrum

Correct answer: Brainstem

The brainstem is a vital part of the brain that connects the cerebrum and cerebellum to the spinal cord, acting as a crucial relay center for motor and sensory information. It contains nuclei and pathways that are essential for maintaining posture, equilibrium, and coordinating basic movements. While the cerebellum refines complex motor skills, the brainstem plays a fundamental role in integrating signals necessary for balance and overall coordination.

Question 11: A switch has two equal-cost paths to the root bridge through different upstream switches. How does STP break this tie?

By using round-robin load balancing
By choosing the path through the switch with the lowest Bridge ID (Correct answer)
By choosing the most recently added path
By comparing port speed

Correct answer: By choosing the path through the switch with the lowest Bridge ID

When root path cost is tied, STP selects the path through the upstream switch with the lowest Bridge ID.

Question 12: What is the function of BPDU Guard in STP?

To automatically configure STP parameters on a switch
To protect against STP BPDUs being sent by unauthorized devices
To prevent BPDUs from being sent on edge ports (Correct answer)
To increase the speed of STP convergence

Correct answer: To prevent BPDUs from being sent on edge ports

BPDU Guard is an STP enhancement designed to protect the STP topology from unauthorized or malicious devices. When enabled on an access port (an edge port connected to an end device), BPDU Guard immediately puts the port into an error-disabled state if it receives a BPDU. This prevents rogue switches from interfering with the STP topology and causing network instability.

Question 13: What is the root bridge in STP?

The fastest switch
The switch with the lowest bridge ID that serves as the reference point for all path calculations (Correct answer)
The newest switch on the network
The switch with the most ports

Correct answer: The switch with the lowest bridge ID that serves as the reference point for all path calculations

The root bridge is elected based on having the lowest bridge ID and serves as the center of the spanning tree topology.

Question 14: What is a broadcast storm in a switched network?

Continuous flooding of broadcast frames caused by a Layer 2 loop, consuming all bandwidth (Correct answer)
A DNS flood attack
A weather event affecting networking
High volume of regular traffic

Correct answer: Continuous flooding of broadcast frames caused by a Layer 2 loop, consuming all bandwidth

A broadcast storm occurs when frames loop endlessly through redundant paths, multiplying exponentially and consuming all network bandwidth.

Question 15: If a switch receives a superior BPDU on its current Root Port, what happens to that port's role?

It immediately enters the Disabled state
It transitions to Designated Port
It may become a Blocked or Alternate Port as STP reconverges (Correct answer)
It remains the Root Port and ignores the BPDU

Correct answer: It may become a Blocked or Alternate Port as STP reconverges

Receiving a superior BPDU triggers STP reconvergence; the current Root Port may lose its role and be blocked if another path to the root becomes preferred.

Question 16: How many seconds does it take for a classic STP port to transition from Blocking to Forwarding by default?

15 seconds
20 seconds
50 seconds (Correct answer)
30 seconds

Correct answer: 50 seconds

A port spends 15 seconds in Listening and 15 seconds in Learning for a total of 30 seconds before entering Forwarding, but max age (20s) is added when recovering from indirect failures, totaling up to 50 seconds.

Question 17: An administrator runs 'spanning-tree vlan 1 priority 32768' on a switch. What is the effect if this is the current root bridge?

The switch immediately transitions all ports to forwarding
The command is rejected because 32768 is not a valid priority multiple
The switch may lose its root bridge role to another switch with a lower priority (Correct answer)
No change occurs because 32768 is already the default

Correct answer: The switch may lose its root bridge role to another switch with a lower priority

Setting priority to 32768 (the default) removes any manually lowered priority advantage, potentially causing another switch with a lower priority to become root.

Question 18: What does the 'spanning-tree loopguard default' command do?

Enables Loop Guard only on root ports
Enables Loop Guard on all PortFast-enabled ports
Disables BPDUs on all ports to prevent loops
Enables Loop Guard globally on all point-to-point ports (Correct answer)

Correct answer: Enables Loop Guard globally on all point-to-point ports

The 'spanning-tree loopguard default' command globally enables Loop Guard on all point-to-point ports, protecting against unidirectional link failures.

Question 19: What is the impact of a suboptimal STP topology?

No impact on performance
Only causes cosmetic issues in monitoring
Traffic takes longer paths, increased latency, underutilized links, and potential congestion (Correct answer)
Only affects specific VLANs

Correct answer: Traffic takes longer paths, increased latency, underutilized links, and potential congestion

A poorly designed STP topology forces traffic along inefficient paths, wasting bandwidth on some links while overloading others.

Question 20: During a network upgrade, a technician accidentally connects a hub between two switches. What is the expected STP port role on the switch port connected to the hub's other side?

Alternate port
Backup port (Correct answer)
Root port
Designated port

Correct answer: Backup port

When two switch ports connect to the same shared-medium segment (e.g., a hub), one becomes the Designated port and the other becomes a Backup port in RSTP.

Question 21: When MST is configured, what is the function of the MST instance 0 (IST)?

It maps only VLAN 1 by default
It runs PVST+ for backward compatibility inside the MST region
It carries all unassigned VLANs and interoperates with CST at region boundaries (Correct answer)
It is reserved for management traffic only

Correct answer: It carries all unassigned VLANs and interoperates with CST at region boundaries

MST Instance 0 (IST) carries all VLANs not mapped to other MSTIs and represents the MST region to the outside CST.

Question 22: In STP, what port type does the designated port face?

Away from the root bridge (downstream) (Correct answer)
Toward the root bridge
Toward a non-STP-capable device
Toward another designated port

Correct answer: Away from the root bridge (downstream)

Designated ports face downstream segments or end devices and are responsible for forwarding traffic away from the root bridge.

Question 23: What is the effect of configuring PortFast on a switch port?

It allows the port to transition immediately to the Forwarding state (Correct answer)
It prevents the port from participating in STP
It increases the time the port spends in the Listening state
It decreases the port’s bandwidth usage

Correct answer: It allows the port to transition immediately to the Forwarding state

PortFast is an STP feature designed for access ports connected to end devices (like workstations or servers), not other switches. When enabled, PortFast allows the port to bypass the Listening and Learning states and immediately transition to the Forwarding state. This significantly reduces the time end devices wait to connect to the network, improving user experience.

Question 24: Which STP port state forwards data frames AND learns MAC addresses?

Blocking
Learning
Listening
Forwarding (Correct answer)

Correct answer: Forwarding

Only the Forwarding state both learns MAC addresses and forwards data frames across the network.

Question 25: A non-root switch has two paths to the root bridge with equal cost. The first path uses a port with priority 128 and port number 1; the second uses priority 128 and port number 2. Which port becomes the Root Port?

Neither port; the switch becomes the root
Port 1, because lower port ID wins (Correct answer)
Both ports become Root Ports
Port 2, because higher port numbers win

Correct answer: Port 1, because lower port ID wins

When path cost and port priority are equal, STP selects the port with the lower port number, making Port 1 the Root Port.

Question 26: What is the function of red blood cells?

Digest nutrients
Transport oxygen (Correct answer)
Fight infection
Produce hormones

Correct answer: Transport oxygen

Red blood cells, also known as erythrocytes, are specialized cells in the blood that contain hemoglobin. Hemoglobin is a protein that binds to oxygen in the lungs and releases it in the tissues throughout the body. Their primary function is to efficiently transport oxygen from the lungs to all body tissues and carry carbon dioxide back to the lungs for exhalation.

Question 27: An administrator wants to influence STP path selection on a per-port basis without changing bridge priority. What is the correct approach?

Enable BPDU filter on competing ports
Change the port cost using 'spanning-tree vlan X cost' (Correct answer)
Use 'spanning-tree mode rapid-pvst' to force path changes
Adjust the bridge priority for the VLAN

Correct answer: Change the port cost using 'spanning-tree vlan X cost'

Adjusting the port cost with 'spanning-tree vlan X cost' influences which path STP prefers without altering the bridge priority.

Question 28: In a multi-VLAN environment using PVST+, what is the operational impact of having too many VLANs each running their own STP instance?

Faster convergence because each VLAN converges independently
Reduced memory usage due to shared BPDU processing
Excessive CPU usage on switches due to processing many STP instances simultaneously (Correct answer)
Automatic load balancing without any configuration

Correct answer: Excessive CPU usage on switches due to processing many STP instances simultaneously

Each PVST+ instance consumes CPU and memory resources; running hundreds of STP instances on a switch can cause CPU spikes and degrade switch performance.

Question 29: In IEEE 802.1D, a non-designated port that is in the Blocking state is sometimes called what?

Passive Port
Backup Port
Alternate Port
Non-Designated Port (Correct answer)

Correct answer: Non-Designated Port

In 802.1D terminology, ports that are not the Root Port or a Designated Port are called Non-Designated Ports and are placed in the Blocking state.

Question 30: Which STP feature automatically sets an access port to forwarding state immediately without going through listening and learning?

BPDU Filter
Root Guard
Loop Guard
PortFast (Correct answer)

Correct answer: PortFast

PortFast bypasses the listening and learning states and immediately transitions an access port to forwarding, reducing convergence time for end devices.

Question 31: What is MSTP (Multiple Spanning Tree Protocol)?

A protocol for multiple networks
A multicast protocol
IEEE 802.1s, which maps multiple VLANs to a reduced number of spanning tree instances (Correct answer)
A backup spanning tree

Correct answer: IEEE 802.1s, which maps multiple VLANs to a reduced number of spanning tree instances

MSTP reduces resource usage by mapping multiple VLANs to fewer spanning tree instances while still providing per-VLAN path optimization.

Question 32: Which STP port role is used to block traffic and prevent switching loops on non-root, non-designated ports?

Alternate port (Correct answer)
Backup port
Designated port
Root port

Correct answer: Alternate port

In RSTP, the alternate port is the blocking port that provides a standby path to the root bridge.

Question 33: What is the best practice for STP timer configuration?

Timer changes are required for all networks
Use default timers unless there is a specific justified reason to change them (Correct answer)
Always increase all timers
Set all timers to minimum

Correct answer: Use default timers unless there is a specific justified reason to change them

Default STP timers are carefully calculated based on worst-case scenarios and should generally not be modified.

Question 34: How do you verify the current STP root bridge?

Check the physical labels on switches
Use ping to find the root
Use show spanning-tree command to display root bridge ID, priority, and port roles (Correct answer)
Look at the IP routing table

Correct answer: Use show spanning-tree command to display root bridge ID, priority, and port roles

The show spanning-tree command displays the root bridge information, local bridge details, and port states and roles.

Question 35: What is a key consideration when implementing STP in a network with a mix of older and newer devices?

Configuring STP to ignore BPDUs from older devices
Using a single STP instance for the entire network
Ensuring all devices support the same version of STP (Correct answer)
Adjusting STP timers to accommodate slower devices

Correct answer: Ensuring all devices support the same version of STP

When implementing STP in a network with a mix of older and newer devices, it is critical to ensure that all devices support and are configured to use the same version of STP. Incompatible STP versions can lead to unpredictable behavior, such as incorrect root bridge elections, blocked links that should be active, or even network loops. Consistent STP versioning across all devices is essential for a stable and reliable spanning tree operation.

Question 36: What is the purpose of configuring a port’s cost in STP?

To control the frequency of BPDU transmissions
To determine the path preference and loop prevention (Correct answer)
To increase the speed of port transitions
To influence the selection of the Root Bridge

Correct answer: To determine the path preference and loop prevention

Port cost is an STP parameter that represents the 'cost' of sending data out of a specific port. Switches use port costs to calculate the shortest path to the Root Bridge and to determine which redundant links should be blocked to prevent loops. Lower cost paths are preferred, allowing administrators to influence traffic flow and ensure optimal network performance and redundancy.

Question 37: A network design requires STP to protect against a downstream switch falsely claiming to be the root bridge. Which feature should be applied to the uplink ports of distribution switches?

PortFast
Root Guard (Correct answer)
BPDU Guard
Loop Guard

Correct answer: Root Guard

Root Guard on distribution switch uplinks prevents any switch connected downstream from becoming the root bridge by blocking ports that send superior BPDUs.

Question 38: What is the STP path cost?

The price of the network cable
A value assigned to each port based on link speed, used to determine the best path to the root bridge (Correct answer)
The physical distance between switches
The number of VLANs on a link

Correct answer: A value assigned to each port based on link speed, used to determine the best path to the root bridge

Path cost is inversely related to link speed - faster links have lower costs, making them preferred paths.

Question 39: What is the purpose of STP path cost manipulation?

To influence which paths STP selects as primary forwarding paths by adjusting port costs (Correct answer)
To modify transmission speed
To change the physical cable routing
To reduce power consumption

Correct answer: To influence which paths STP selects as primary forwarding paths by adjusting port costs

By adjusting port costs, network administrators can make STP prefer certain paths over others, optimizing traffic flow according to network design goals.

Question 40: Which STP timer controls how long a port remains in the listening and learning states?

Max age
Hello time
Forward delay (Correct answer)
Hold time

Correct answer: Forward delay

Forward delay (default 15 seconds) is the time a port spends in each of the listening and learning states before transitioning to forwarding.

Question 41: Which STP design practice reduces convergence time in large Layer 2 networks by limiting the STP diameter?

Enabling BPDU Filter on all trunk ports
Using TCN BPDUs exclusively
Setting all switches to the same bridge priority
Keeping the network diameter to 7 hops or fewer (Correct answer)

Correct answer: Keeping the network diameter to 7 hops or fewer

STP's default timer calculations assume a maximum network diameter of 7 switches; exceeding this can cause timing issues and topology instability.

Question 42: What happens when a root bridge fails in STP?

A new root bridge election occurs using the bridge with the next lowest bridge ID (Correct answer)
Manual intervention is always required
All ports go to blocking
The network permanently stops

Correct answer: A new root bridge election occurs using the bridge with the next lowest bridge ID

When the root bridge fails, the network automatically elects a new root bridge based on the next lowest bridge ID.

Question 43: Which RSTP port role is equivalent to the 802.1D blocking port and does not forward traffic but can quickly transition to forwarding?

Disabled port
Backup port
Discarding port
Alternate port (Correct answer)

Correct answer: Alternate port

The RSTP alternate port provides an alternate path to the root and can transition to root port role quickly if the current root port fails.

Question 44: What is the Bridge Protocol Data Unit (BPDU) guard feature designed to do?

Increase BPDU transmission frequency
Prevent root bridge elections on trunk ports
Shut down a PortFast port if it receives a BPDU (Correct answer)
Prioritize BPDU traffic over data frames

Correct answer: Shut down a PortFast port if it receives a BPDU

BPDU Guard err-disables a PortFast-enabled port if any BPDU is received, protecting the topology from rogue switches.

Question 45: How does Rapid STP (802.1w) achieve faster convergence compared to 802.1D?

It reduces the default forward delay to 1 second
It bypasses the root bridge election process
It eliminates the hello timer entirely
It uses a proposal/agreement handshake to transition ports to forwarding without waiting for timers (Correct answer)

Correct answer: It uses a proposal/agreement handshake to transition ports to forwarding without waiting for timers

RSTP uses a proposal/agreement mechanism between directly connected switches to synchronize port roles and states without relying on timer expiry.

Question 46: What command enables PortFast on all non-trunking access ports globally on a Cisco switch?

spanning-tree portfast enable global
spanning-tree portfast default (Correct answer)
spanning-tree portfast all
interface range fastethernet 0/1-24 spanning-tree portfast

Correct answer: spanning-tree portfast default

The global command 'spanning-tree portfast default' enables PortFast on all non-trunking interfaces without configuring each one individually.

Question 47: What is edge port in RSTP?

A port at the network edge router
A port with the highest priority
A port connected to an end device that transitions immediately to forwarding state (Correct answer)
A port connecting to another switch

Correct answer: A port connected to an end device that transitions immediately to forwarding state

Edge ports (equivalent to PortFast) are connected to end devices and can immediately forward traffic without risking loops.

Question 48: Which MSTP concept describes the role of an MST region boundary port connecting to another region or a legacy STP bridge?

IST Master Port
Regional Root Port
Boundary Port (Correct answer)
Edge Port

Correct answer: Boundary Port

A boundary port is an MST port that connects to a switch in a different MST region or running legacy STP/RSTP.

Question 49: Which STP timer controls how long a port waits in the Listening and Learning states?

Max age
Hello time
Forward delay (Correct answer)
Hold time

Correct answer: Forward delay

The forward delay timer (default 15 seconds) determines the duration spent in each of the Listening and Learning states.

Question 50: When designing a network with STP, what is the primary reason for creating redundant paths between switches?

To ensure high availability and fault tolerance (Correct answer)
To simplify network management
To increase network bandwidth
To enhance network security

Correct answer: To ensure high availability and fault tolerance

Redundant paths are crucial in network design to provide alternative routes for data in case a primary link or device fails. While Spanning Tree Protocol (STP) blocks these redundant paths to prevent loops, they are immediately activated upon a failure. This ensures continuous network operation, minimizing downtime and significantly enhancing overall network availability and fault tolerance.

Question 51: What happens when PortFast is incorrectly enabled on a switch-to-switch link?

Nothing happens
The port operates more reliably
It can cause temporary bridging loops because the port skips listening and learning states (Correct answer)
The link becomes faster

Correct answer: It can cause temporary bridging loops because the port skips listening and learning states

Enabling PortFast on inter-switch links bypasses STP protection, potentially causing loops during topology changes since the port immediately forwards traffic.

Question 52: Which configuration error would cause two switches running PVST+ to end up with different root bridges for the same VLAN?

Mismatched port costs on access ports
Both switches using the default priority of 32768 with different MAC addresses
One switch having 'spanning-tree vlan 10 priority 0' and the other having 'spanning-tree vlan 10 priority 4096' (Correct answer)
Mismatched duplex settings on the trunk link

Correct answer: One switch having 'spanning-tree vlan 10 priority 0' and the other having 'spanning-tree vlan 10 priority 4096'

If one switch has priority 0 and another has any higher priority, the switch with priority 0 wins the root election — but if the intent was both to be root, the configuration is conflicting.

STP Spanning Tree Protocol Certification Exam

The STP Spanning Tree Protocol Certification Exam covers STP fundamentals and concepts, configuration and management, advanced features and variants (RSTP, MSTP, PVST+), network design and implementation, and troubleshooting loop-free Layer 2 networks.

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60 min

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70%

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