CCNA Spanning Tree Questions

Multi-Selecteasy

Which two statements about PortFast are true? (Choose two.)

Select 2 answers

A.PortFast allows a port to transition directly from blocking to forwarding state.

B.PortFast should be enabled on ports that connect to end-user devices.

C.PortFast disables BPDU processing on the port.

D.PortFast is automatically enabled on all trunk ports.

E.PortFast changes the root bridge election process.

AnswersA, B

Correct because PortFast bypasses the listening and learning states, enabling immediate forwarding.

Why this answer

PortFast is a Cisco enhancement that allows a port to transition immediately from blocking to forwarding, bypassing the listening and learning states. It is intended for ports connected to end hosts, not to other switches. When PortFast is enabled, the port still participates in STP BPDU processing, but the BPDU guard feature can be used to protect against accidental loops.

PortFast does not affect the root bridge election or the designated port selection process.

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Multi-Selecthard

Which three statements about the Multiple Spanning Tree Protocol (MSTP) are true? (Choose three.)

Select 3 answers

A.MSTP allows multiple VLANs to be mapped to a single spanning-tree instance.

B.MSTP uses an Internal Spanning Tree (IST) to interconnect MST regions.

C.MSTP is backward compatible with 802.1D and RSTP.

D.MSTP requires a separate spanning-tree instance for every VLAN.

E.MSTP uses a different BPDU format than RSTP.

AnswersA, B, C

Correct because MSTP reduces the number of STP instances by grouping VLANs, improving scalability.

Why this answer

MSTP (802.1s) allows mapping multiple VLANs to a single spanning-tree instance, reducing the number of STP instances needed. It uses an internal spanning tree (IST) to interconnect MST regions. MSTP is backward compatible with 802.1D and RSTP through the use of common spanning tree (CST) at the region boundary.

MSTP does not require a separate instance for every VLAN like PVST+; it groups VLANs into instances. MSTP uses the same BPDU format as RSTP with additional MST-specific information.

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MCQhard

A network engineer runs the following command on Switch SW1: SW1# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Gi0/2 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-1005 Gi0/2 1-1005 Port Vlans allowed and active in management domain Gi0/1 1,10,20 Gi0/2 1,10,20 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1,10,20 Gi0/2 1,10,20 Based on this output, what can be concluded?

A.VLANs 2-9 are allowed but not active on the trunk.

B.The trunk is using ISL encapsulation.

C.VLAN 1 is pruned from the trunk.

D.Only VLANs 10 and 20 are forwarding traffic.

AnswerA

The 'allowed' list includes 1-1005, but only VLANs 1,10,20 are active; thus VLANs 2-9 are allowed but not active (not created in VLAN database).

Why this answer

The output shows that VLANs 1-1005 are allowed on the trunk, but only VLANs 1, 10, and 20 are listed as active in the management domain. This means VLANs 2-9 are configured on the trunk but are not active (i.e., not created or not present on the switch), so they do not forward traffic. Option A correctly identifies this condition.

Exam trap

Cisco often tests the difference between 'allowed on trunk' and 'active in management domain' to trick candidates into thinking all allowed VLANs are forwarding, when in fact only active VLANs forward traffic.

How to eliminate wrong answers

Option B is wrong because the encapsulation is explicitly shown as '802.1q', not ISL, which is a Cisco proprietary protocol that is now largely deprecated. Option C is wrong because VLAN 1 is listed in the 'Vlans in spanning tree forwarding state and not pruned' section, indicating it is forwarding and not pruned; pruning would remove it from that list. Option D is wrong because VLAN 1 is also in the forwarding state and not pruned, so traffic for VLAN 1 is also being forwarded, not just VLANs 10 and 20.

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MCQmedium

Given the following configuration snippet on a Cisco IOS-XE switch: interface GigabitEthernet1/0/1 switchport mode access switchport access vlan 10 spanning-tree portfast monitor session 1 source interface GigabitEthernet1/0/1 both monitor session 1 destination interface GigabitEthernet1/0/2 What is the effect of this configuration?

A.All traffic entering and leaving GigabitEthernet1/0/1 is copied to GigabitEthernet1/0/2.

B.Only traffic entering GigabitEthernet1/0/1 is copied to GigabitEthernet1/0/2.

C.Traffic on GigabitEthernet1/0/2 is replicated to GigabitEthernet1/0/1.

D.The configuration is invalid because the destination port must be in trunk mode.

AnswerA

The 'both' keyword copies both ingress and egress traffic from the source interface to the destination.

Why this answer

The configuration uses a local SPAN session to copy traffic from a source interface (GigabitEthernet1/0/1) to a destination interface (GigabitEthernet1/0/2). The keyword 'both' specifies that both ingress and egress traffic on the source port are mirrored, so all traffic entering and leaving GigabitEthernet1/0/1 is sent to the destination port for monitoring.

Exam trap

The trap here is that candidates often confuse 'both' with 'rx' or 'tx' and assume only one direction is mirrored, or they mistakenly think the destination port must be in trunk mode to carry VLAN tags, but in local SPAN the destination port can be an access port and the mirrored frames are sent untagged by default.

How to eliminate wrong answers

Option B is wrong because it claims only ingress traffic is copied, but the 'both' keyword explicitly includes egress traffic as well. Option C is wrong because it reverses the direction of the SPAN session, stating traffic from the destination is replicated to the source, which is not how SPAN works; the source is always the monitored port. Option D is wrong because the destination port in a local SPAN session does not need to be in trunk mode; it can be an access port, and the configuration is valid as long as the destination port is not used for normal data forwarding.

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Matchingmedium

Match each Spanning Tree Protocol (STP) variant to its key characteristic.

Drag a concept onto its matching description — or click a concept then click the description.

Concepts

Matches

Original standard, slow convergence

Fast convergence, backward compatible

Multiple spanning trees per VLAN group

Cisco proprietary, per-VLAN STP

Cisco proprietary, per-VLAN RSTP

Why these pairings

STP (802.1D) is the original slow-converging standard; RSTP (802.1w) provides rapid convergence; MSTP (802.1s) maps multiple VLANs to fewer spanning tree instances; PVST+ runs per-VLAN STP. Common confusions include equating STP with per-VLAN and RSTP with multiple instances.

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MCQmedium

Examine the following partial Cisco IOS-XE configuration: interface GigabitEthernet0/1 switchport mode access switchport access vlan 10 ip access-group ACL_IN in spanning-tree portfast What is the effect of this configuration?

A.The port will immediately transition to forwarding state, reducing STP convergence time for end hosts.

B.The port will become a trunk port and participate in VLAN trunking.

C.The port will use Rapid PVST+ and immediately forward after a link failure.

D.The port will block all inbound traffic due to the ACL.

AnswerA

spanning-tree portfast on an access port causes immediate forwarding, as intended for host ports.

Why this answer

The configuration enables PortFast on an access port, allowing it to transition directly to forwarding state, bypassing the listening and learning phases. This is commonly used for end-host ports to avoid delays caused by spanning-tree convergence.

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Drag & Dropmedium

Drag and drop the steps of STP portfast and BPDU guard configuration into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps

Order

1Step 1

2Step 2

3Step 3

4Step 4

5Step 5

Why this order

PortFast is configured on an access port to immediately transition to forwarding. BPDU guard is then enabled to protect against rogue BPDUs. The configuration is applied at the interface level, and verification ensures the port is in forwarding state.

If a BPDU is received, BPDU guard errdisables the port.

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MCQmedium

A network engineer runs the following command on Switch SW2: SW2# show spanning-tree vlan 20 VLAN0020 Spanning tree enabled protocol ieee Root ID Priority 24596 Address aabb.cc00.0200 Cost 4 Port 1 (GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32778 (priority 32768 sys-id-ext 20) Address aabb.cc00.0300 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- ------------------------------ Gi0/1 Root FWD 4 128.1 P2p Gi0/2 Desg FWD 4 128.2 P2p Gi0/3 Altn BLK 4 128.3 P2p Based on this output, what can be concluded?

A.SW2 is the root bridge for VLAN 20.

B.GigabitEthernet0/3 is in the Blocking state to prevent a loop.

C.The root port cost is 8.

D.SW2's bridge priority is 32768.

AnswerB

Correct. Gi0/3 is shown as Altn BLK (Alternate, Blocking), which prevents a loop.

Why this answer

The output shows that SW2's Gi0/3 is in the Alternate (Altn) role and Blocking (BLK) state. In Rapid PVST+ (IEEE 802.1w), an alternate port provides a backup path to the root bridge and is placed in a blocking state to prevent Layer 2 loops. This confirms that Gi0/3 is blocking to maintain a loop-free topology.

Exam trap

Cisco often tests the distinction between the root bridge's priority and a non-root switch's priority, including the sys-id-ext addition, so candidates mistakenly think the bridge priority is the base priority without adding the VLAN ID.

How to eliminate wrong answers

Option A is wrong because SW2 is not the root bridge; the Root ID shows priority 24596 and address aabb.cc00.0200, while SW2's Bridge ID has priority 32778 and address aabb.cc00.0300, so SW2 is a non-root switch. Option C is wrong because the root port cost is explicitly shown as 4 (under the Root ID section, Cost 4), not 8. Option D is wrong because SW2's bridge priority is 32778 (priority 32768 + sys-id-ext 20), not 32768; the sys-id-ext (VLAN ID) is added to the base priority.

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MCQmedium

spanning-tree mode rapid-pvst What is the effect of this global configuration command?

A.The switch will use Rapid PVST+ for all VLANs, providing faster convergence than classic STP.

B.The switch will use MSTP for all VLANs.

C.The switch will use classic STP for all VLANs.

D.The switch will disable STP on all ports.

AnswerA

Rapid PVST+ is the Cisco implementation of RSTP per VLAN.

Why this answer

The command 'spanning-tree mode rapid-pvst' enables Rapid PVST+ (Per-VLAN Spanning Tree Plus) on the switch, which runs a separate instance of RSTP (802.1w) for each VLAN. This provides faster convergence than classic STP (802.1D) by using mechanisms such as sync/agreement handshakes, edge ports, and link types, while still maintaining per-VLAN topology independence.

Exam trap

Cisco often tests the distinction between 'rapid-pvst' (RSTP per VLAN) and 'mst' (MSTP, which maps multiple VLANs to fewer instances), and candidates may confuse 'rapid-pvst' with simply enabling RSTP globally without understanding it applies per VLAN.

How to eliminate wrong answers

Option B is wrong because MSTP (Multiple Spanning Tree Protocol, 802.1s) is enabled with the command 'spanning-tree mode mst', not 'rapid-pvst'. Option C is wrong because classic STP (802.1D) is the default mode on many switches or is set with 'spanning-tree mode pvst', not with 'rapid-pvst'. Option D is wrong because the command does not disable STP; STP is disabled globally with 'no spanning-tree vlan <vlan>' or 'spanning-tree mode none' (if supported), not by setting the mode to rapid-pvst.

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Multi-Selecthard

Which three statements about Multiple Spanning Tree Protocol (MSTP) are true? (Choose three.)

Select 3 answers

A.MSTP allows multiple VLANs to be grouped into a single spanning-tree instance, reducing CPU and memory usage.

B.In MSTP, the Internal Spanning Tree (IST) instance is instance 0 and is always present in every MST region.

C.MSTP requires that all switches in the same MST region have the same VLAN-to-instance mapping, revision number, and region name.

D.MSTP automatically load-balances traffic across all available uplinks without any configuration.

E.MSTP requires a separate root bridge to be elected for each VLAN in the network.

AnswersA, B, C

Correct. This is the primary benefit of MSTP: mapping many VLANs to fewer STP instances.

Why this answer

MSTP (IEEE 802.1s) allows multiple VLANs to be mapped to a single spanning-tree instance (MST instance), reducing the number of STP instances needed. It uses an Internal Spanning Tree (IST) instance (instance 0) that always runs and carries BPDUs for the region. Switches in the same MST region must have identical VLAN-to-instance mappings, revision number, and region name.

MSTP interoperates with Rapid PVST+ at region boundaries by using PVST simulation mode. MSTP does not require a separate root bridge for each VLAN; instead, each MST instance has its own root bridge.

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Drag & Dropmedium

Drag and drop the steps of STP root bridge election and port state transitions into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps

Order

1Step 1

2Step 2

3Step 3

4Step 4

5Step 5

Why this order

STP first elects a root bridge based on lowest bridge ID. Then each non-root bridge selects one root port (lowest path cost to root). Next, each segment elects a designated port (lowest root path cost, then lowest bridge ID).

All other ports become alternate (blocking). Finally, ports transition through blocking, listening, learning, and forwarding states.

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Drag & Dropmedium

Drag and drop the steps of STP portfast and BPDU guard configuration into the correct order, from first to last.

Drag steps to the numbered slots on the right, or tap a step then tap a slot.

Steps

Order

1Step 1

2Step 2

3Step 3

4Step 4

5Step 5

Why this order

PortFast is enabled globally or per interface to bypass listening/learning. BPDU guard is then configured to disable the port if a BPDU is received. The configuration is applied to the interface, and the port transitions to forwarding immediately.

Finally, errdisable recovery can be set.

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