STP Spanning Tree Protocol Certification Exam — Questions and Answers

Question 1: In STP, what port type does the designated port face?

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

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 2: What is a BPDU guard?

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

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 3: What is the default STP hello time on Cisco switches?

10 seconds
4 seconds
1 second
2 seconds (Correct answer)

Correct answer: 2 seconds

The default STP hello time is 2 seconds, which is the interval between configuration BPDUs sent by the root bridge.

Question 4: What is the purpose of the Topology Change Notification (TCN) BPDU in STP?

To advertise port costs
To elect a new root bridge
To signal that the network topology has changed (Correct answer)
To reset bridge priorities

Correct answer: To signal that the network topology has changed

A TCN BPDU is sent upstream toward the root bridge to notify all switches that a topology change has occurred, prompting MAC table flushes.

Question 5: Which combination of port state and role correctly describes a port that is actively forwarding data on the root bridge?

Learning / Root Port
Forwarding / Designated Port (Correct answer)
Forwarding / Root Port
Blocking / Designated Port

Correct answer: Forwarding / Designated Port

All ports on the root bridge are Designated Ports and, when active, they are in the Forwarding state.

Question 6: In an MSTP design, what is the purpose of a Common and Internal Spanning Tree (CIST)?

It interconnects multiple MST regions and interoperates with 802.1D bridges (Correct answer)
It provides per-VLAN spanning tree for legacy switches
It acts as a backup to the IST inside each MST region
It replaces PVST+ in Cisco proprietary implementations

Correct answer: It interconnects multiple MST regions and interoperates with 802.1D bridges

The CIST spans all switches in all MST regions and interoperates with 802.1D/PVST+ bridges, forming the backbone of MSTP topology.

Question 7: What happens when a root bridge fails in STP?

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

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

No change occurs because 32768 is already the default
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)
The switch immediately transitions all ports to forwarding

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 9: Which device role in STP is responsible for forwarding traffic to all other switches in the network segment?

Root Port
Blocking Port
Designated Port (Correct answer)
Root Bridge

Correct answer: Designated Port

A Designated Port is a non-root port on a segment that is closest to the Root Bridge in terms of path cost. It is responsible for forwarding traffic to and from that segment towards the Root Bridge and other parts of the network. Every segment in an STP-enabled network must have one and only one Designated Port to ensure efficient traffic flow and prevent loops.

Question 10: A network design requires fast failover (under 1 second) when a distribution switch uplink fails. Which STP configuration best achieves this goal?

MSTP with a single region spanning all switches
PVST+ with Backbone Fast and Uplink Fast enabled
RSTP with pre-configured Alternate ports on access switches (Correct answer)
Classic STP with reduced Forward Delay to 1 second

Correct answer: RSTP with pre-configured Alternate ports on access switches

RSTP's Alternate port can transition to Forwarding almost immediately upon root port failure, achieving sub-second convergence without timer dependencies.

Question 11: Which command is used on a Cisco switch to manually set a switch as the root bridge using a specific priority?

spanning-tree root-bridge vlan 1
spanning-tree mode root primary
switchport spanning-tree priority 0
spanning-tree vlan 1 priority 0 (Correct answer)

Correct answer: spanning-tree vlan 1 priority 0

Setting spanning-tree vlan 1 priority 0 gives the switch the lowest possible bridge priority, making it the root bridge.

Question 12: What is Rapid Spanning Tree Protocol (RSTP)?

A completely different protocol
A slower version of STP
An encryption protocol
IEEE 802.1w, an evolution of STP providing faster convergence (1-2 seconds) (Correct answer)

Correct answer: IEEE 802.1w, an evolution of STP providing faster convergence (1-2 seconds)

RSTP dramatically reduces convergence time from 30-50 seconds to 1-2 seconds through improved state transitions.

Question 13: What is the root bridge in STP?

The switch with the most ports
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 fastest switch

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 three parameters must match exactly for two switches to be in the same MST region?

Region name, hello time, and bridge priority
Region name, revision number, and VLAN-to-instance mapping (Correct answer)
Revision number, bridge MAC, and VLAN-to-instance mapping
Region name, bridge priority, and VLAN list

Correct answer: Region name, revision number, and VLAN-to-instance mapping

MST region membership requires identical configuration name, revision number, and VLAN-to-instance mapping on all participating switches.

Question 15: When comparing Rapid PVST+ to MSTP, which statement is accurate?

Rapid PVST+ and MSTP produce identical BPDU counts regardless of VLAN count
MSTP is more scalable because it groups VLANs into instances, reducing the number of spanning tree calculations (Correct answer)
MSTP cannot interoperate with Rapid PVST+ under any configuration
Rapid PVST+ scales better in large VLAN environments because it uses fewer BPDUs

Correct answer: MSTP is more scalable because it groups VLANs into instances, reducing the number of spanning tree calculations

MSTP reduces CPU and memory overhead by mapping many VLANs to a single instance, whereas Rapid PVST+ runs one STP process per VLAN.

Question 16: Which STP feature automatically sets an access port to forwarding state immediately without going through listening and learning?

BPDU Filter
PortFast (Correct answer)
Loop Guard
Root Guard

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 17: What is the primary advantage of MSTP (Multiple Spanning Tree Protocol) over PVST+?

It eliminates the need for a root bridge
Faster convergence per VLAN
It supports more than 4096 VLANs
Fewer STP instances by mapping multiple VLANs to one instance (Correct answer)

Correct answer: Fewer STP instances by mapping multiple VLANs to one instance

MSTP maps multiple VLANs to a single spanning tree instance, reducing CPU and memory overhead compared to running one instance per VLAN.

Question 18: What is the recommended practice when enabling BPDU Guard globally via 'spanning-tree portfast bpduguard default'?

Apply it only on trunk ports to prevent rogue switches
Ensure PortFast is also enabled globally so BPDU Guard activates only on PortFast ports (Correct answer)
Enable it on root ports to prevent topology changes
Disable it on all uplink ports manually afterward

Correct answer: Ensure PortFast is also enabled globally so BPDU Guard activates only on PortFast ports

When BPDU Guard is enabled globally, it activates only on PortFast-enabled interfaces, so PortFast must also be enabled globally or per-port for BPDU Guard to take effect.

Question 19: What is the TCN BPDU?

A type of network attack
A management message
A timer configuration
Topology Change Notification sent by a switch when it detects a port state change (Correct answer)

Correct answer: Topology Change Notification sent by a switch when it detects a port state change

TCN BPDUs notify the root bridge of topology changes so the network can update its forwarding tables.

Question 20: What is the purpose of configuring a port’s cost in STP?

To increase the speed of port transitions
To control the frequency of BPDU transmissions
To determine the path preference and loop prevention (Correct answer)
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 21: What is a key consideration when implementing STP in a network with a mix of older and newer devices?

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

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 22: Which STP port state immediately precedes the Forwarding state during normal port initialization?

Blocking
Disabled
Listening
Learning (Correct answer)

Correct answer: Learning

A port transitions from Listening → Learning → Forwarding; the Learning state is the final intermediate state before a port begins forwarding data.

Question 23: What is the STP path cost?

The number of VLANs on a link
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

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 24: 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
Mismatched duplex settings on the trunk link
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)

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.

Question 25: What is the STP port cost for a 1 Gbps link using the original IEEE cost values?

4 (Correct answer)
100
1
19

Correct answer: 4

The original IEEE 802.1D cost for a 1 Gbps link is 4, based on the formula 1000/bandwidth in Mbps.

Question 26: What happens when PortFast is incorrectly enabled on a switch-to-switch link?

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

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 27: A switch port remains in blocking state even though it should be forwarding. Which tool helps identify whether Root Guard or Loop Guard is the cause?

show interfaces trunk
show spanning-tree vlan 1 detail
show spanning-tree inconsistentports (Correct answer)
debug spanning-tree events

Correct answer: show spanning-tree inconsistentports

The 'show spanning-tree inconsistentports' command lists all ports currently in a root-inconsistent or loop-inconsistent state caused by Root Guard or Loop Guard.

Question 28: Which STP variant was specifically designed to reduce convergence time to under one second by using a proposal/agreement handshake?

RSTP (802.1w) (Correct answer)
PVST+
MSTP (802.1s)
Classic STP (802.1D)

Correct answer: RSTP (802.1w)

RSTP uses a proposal/agreement mechanism between adjacent switches to achieve near-instant convergence without relying on timers.

Question 29: Which STP enhancement runs a separate spanning tree instance per VLAN on Cisco switches?

MSTP (802.1s)
Common Spanning Tree (CST)
RSTP (802.1w)
PVST+ (Correct answer)

Correct answer: PVST+

Per-VLAN Spanning Tree Plus (PVST+) runs an independent STP instance for each VLAN, allowing load balancing across redundant links.

Question 30: What command enables PortFast on all non-trunking access ports globally on a Cisco switch?

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

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 31: What is an alternate port in RSTP?

A backup to the root port that can immediately take over if the root port fails (Correct answer)
A port that alternates between states
A port for alternate VLANs
A port connected to an alternate network

Correct answer: A backup to the root port that can immediately take over if the root port fails

The alternate port provides a pre-calculated backup path to the root bridge, enabling rapid failover.

Question 32: Which RSTP port role is equivalent to the 802.1D blocking port and does not forward traffic but can quickly transition to forwarding?

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

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 33: In a network with 48-port access switches uplinked via Gigabit Ethernet to distribution switches, what is the default STP port cost for these Gigabit uplinks using the IEEE long method?

200000
20000 (Correct answer)
4
19

Correct answer: 20000

Using the IEEE 802.1t long path cost method, a 1 Gbps link has a default cost of 20000, providing finer granularity than the original short method.

Question 34: When two switches are connected by multiple parallel links and STP is running, what happens to the redundant links without additional configuration?

STP blocks all but one link to prevent loops, wasting bandwidth (Correct answer)
Links are automatically aggregated into an EtherChannel
All links forward traffic in round-robin fashion
Each link is assigned to a different VLAN automatically

Correct answer: STP blocks all but one link to prevent loops, wasting bandwidth

STP will block all but the single best-cost path between two switches, leaving redundant links idle unless EtherChannel or MSTP load-balancing is configured.

Question 35: An administrator wants to influence STP path selection on a per-port basis without changing bridge priority. What is the correct approach?

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

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 36: What is the best practice for STP timer configuration?

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

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 37: What is the STP max age timer?

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

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 38: What is the purpose of STP path cost manipulation?

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

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 39: How does the network topology affect STP operation and design?

It influences the election of the Root Bridge and path cost calculations (Correct answer)
It sets the time intervals for data retransmission
It determines the maximum bandwidth available to each switch
It dictates the encryption methods used for data transmission

Correct answer: It influences the election of the Root Bridge and path cost calculations

The physical arrangement of switches and links in a network directly impacts how STP operates. The network topology dictates which switch will be elected as the Root Bridge, based on its Bridge ID, and influences the calculation of path costs to reach the Root Bridge. These factors are fundamental in determining which links will be active and which will be blocked to create a loop-free topology.

Question 40: What does MSTP (802.1s) allow that RSTP alone does not?

Faster convergence than RSTP
Dynamic VLAN assignment based on MAC addresses
Port authentication before STP participation
Multiple VLANs mapped to different spanning tree instances (Correct answer)

Correct answer: Multiple VLANs mapped to different spanning tree instances

MSTP allows multiple VLANs to be mapped to separate spanning tree instances, enabling load balancing across redundant links.

Question 41: In a network with dual-homed access switches, which STP port role describes the non-root, non-forwarding port that provides a redundant path to the root bridge?

Alternate port (RSTP) (Correct answer)
Backup port (RSTP)
Non-designated port (STP)
Blocked port (STP)

Correct answer: Alternate port (RSTP)

In RSTP, the Alternate port is a non-forwarding port on a switch that has a better path to the root through a different bridge than its current root port.

Question 42: In IEEE 802.1D, a non-designated port that is in the Blocking state is sometimes called what?

Backup Port
Passive 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 43: How does STP handle a switch receiving superior BPDUs on its root port?

Sends an error message
Ignores the BPDUs
The switch updates its root bridge information and recalculates the spanning tree topology (Correct answer)
Shuts down the port

Correct answer: The switch updates its root bridge information and recalculates the spanning tree topology

When a switch receives a superior BPDU (from a bridge with a lower ID), it updates its understanding of the topology and may select a new root bridge and root port.

Question 44: Which of the following correctly describes the 'spanning-tree pathcost method long' command?

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
Applies only to VLAN 1 path cost calculations

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 45: How does STP interact with Link Aggregation (e.g., EtherChannel) in a network?

STP disables Link Aggregation to prevent loops
STP does not interact with Link Aggregation, as they operate independently
STP requires separate STP instances for each physical link in the aggregation
STP uses the aggregated link as a single logical link when calculating path costs (Correct answer)

Correct answer: STP uses the aggregated link as a single logical link when calculating path costs

STP interacts with Link Aggregation (e.g., EtherChannel) by treating the bundled physical links as a single logical link. This means that STP calculates path costs and makes forwarding decisions based on the aggregated link as a whole, rather than individual member links. This integration prevents STP from blocking individual links within the aggregation, allowing the full bandwidth of the EtherChannel to be utilized while still maintaining a loop-free network topology.

Question 46: What does the STP MaxAge timer control?

The maximum age of MAC table entries
How long a port stays in the Learning state
How long a switch stores BPDU information before discarding it (Correct answer)
How often the root bridge sends Hello BPDUs

Correct answer: How long a switch stores BPDU information before discarding it

MaxAge (default 20 seconds) is the maximum time a switch retains BPDU information before considering the root bridge unreachable.

Question 47: What happens to a port configured with 'spanning-tree bpduguard enable' when it receives a BPDU?

The port sends a topology change notification
The port transitions to blocking state
The port discards the BPDU and continues forwarding
The port is placed in err-disabled state (Correct answer)

Correct answer: The port is placed in err-disabled state

BPDU Guard immediately err-disables a port upon receiving a BPDU to protect against unauthorized switches.

Question 48: In a multi-VLAN environment using PVST+, what is the operational impact of having too many VLANs each running their own STP instance?

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

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 49: What is the recommended practice for STP root bridge placement?

The root bridge does not matter
Place it on an edge switch
Any switch can be root bridge without consequence
Place the root bridge on a high-performance core switch central to the network (Correct answer)

Correct answer: Place the root bridge on a high-performance core switch central to the network

The root bridge should be a centrally located, high-performance switch to optimize traffic paths and minimize spanning tree depth.

Question 50: In STP, what is a Bridge Protocol Data Unit (BPDU)?

A data frame used for routing decisions in a network
A tool used to encrypt data on the network
A message exchanged between switches to share information about network topology (Correct answer)
A type of error message sent during network congestion

Correct answer: A message exchanged between switches to share information about network topology

Bridge Protocol Data Units (BPDUs) are special data frames exchanged between switches participating in STP. These messages contain information such as the Bridge ID, Root Bridge ID, and path cost. BPDUs are crucial for switches to elect the Root Bridge, determine port roles, and ultimately build a loop-free logical topology by sharing network topology information.

Question 51: What is the impact of a suboptimal STP topology?

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

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 52: How does STP interact with EtherChannel?

STP treats an EtherChannel as a single logical link, preventing it from blocking individual member ports (Correct answer)
They are incompatible
STP blocks all EtherChannel ports
EtherChannel disables STP

Correct answer: STP treats an EtherChannel as a single logical link, preventing it from blocking individual member ports

EtherChannel bundles physical links into one logical link, so STP sees a single high-bandwidth connection rather than redundant links.

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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