350-401 Spanning Tree Protocol — Timed 30 Questions

Question 1

A network engineer is troubleshooting an STP issue in a switched network. The network has two distribution switches connected via a trunk, and each distribution switch connects to the same access switch. The engineer notices that the root bridge is not the intended distribution switch. Upon checking, the engineer sees that the access switch has a higher priority than the distribution switches. The engineer needs to ensure that the intended distribution switch becomes the root bridge without causing a temporary loop. What should the engineer do?

Accepted Answer

Configure the 'spanning-tree vlan vlan-id root primary' command on the intended distribution switch.. Option A is correct because the 'spanning-tree vlan vlan-id root primary' command dynamically sets the switch's bridge priority to 24576 (or 4096 if the current root has a priority lower than 24576) and ensures the switch becomes the root bridge without manual priority miscalculation. This command also adjusts the priority of neighboring switches if needed, preventing temporary loops by avoiding the need to disable or reset STP. It is the safest and most efficient method to force a specific switch to become the root bridge in a live network.

Answer

Set the priority of the access switch to 0 using the 'spanning-tree vlan vlan-id priority 0' command.

Answer

Increase the priority of the distribution switch to 61440 using the 'spanning-tree vlan vlan-id priority 61440' command.

Answer

Disable STP on the distribution switch and manually configure it as the root bridge.

Question 2

An engineer is designing a redundant Layer 2 network with multiple VLANs. The network uses Rapid PVST+ for STP. The engineer wants to ensure that different VLANs have different root bridges to optimize traffic flow. The distribution switches are Cisco Catalyst 9300s. The engineer has configured one distribution switch as the root for VLANs 10 and 20, and the other as the root for VLANs 30 and 40. However, after implementation, the engineer notices that all VLANs have the same root bridge. What is the most likely cause?

Accepted Answer

The engineer used the 'spanning-tree root primary' command without specifying the VLAN, which sets the priority for all VLANs.. The 'spanning-tree root primary' command without specifying a VLAN sets the bridge priority to 24576 for all VLANs on the switch, making it the root for every VLAN. Since the engineer intended different root bridges per VLAN, this command overrides any per-VLAN priority settings and causes all VLANs to elect the same root bridge.

Answer

The engineer enabled BPDU guard on all access ports, which prevents the switch from receiving superior BPDUs.

Answer

The engineer enabled PortFast on all trunk ports, which causes the switch to ignore BPDUs and become root.

Answer

The engineer enabled UplinkFast, which forces the switch to become the root bridge for all VLANs.

Question 3

A network engineer is troubleshooting a Layer 2 loop issue. The network consists of three switches: SW1, SW2, and SW3, all connected in a triangle. The engineer notices that SW1 is the root bridge. After a link failure between SW1 and SW2, the network experiences a temporary loop. The engineer wants to prevent such loops in the future by enabling a feature that provides faster convergence and prevents temporary loops during topology changes. The engineer is using Rapid PVST+. Which feature should the engineer enable?

Accepted Answer

Enable Loop Guard on all switch ports.. When a link fails in a triangle topology with Rapid PVST+, the switch that lost its root port may temporarily transition a blocked alternate port to forwarding before the new root port is fully synchronized, causing a loop. Enabling Loop Guard on all switch ports prevents this by keeping a port in a blocking state if BPDUs are not received, ensuring that a port does not erroneously transition to forwarding during a topology change. This provides faster convergence without temporary loops by enforcing BPDU-based loop prevention.

Answer

Enable BPDU Guard on all switch ports.

Answer

Enable Root Guard on all switch ports.

Answer

Enable UDLD on all fiber links.

Question 4

An engineer is configuring a new access switch that connects to two distribution switches via trunk links. The distribution switches are configured with Rapid PVST+ and are both running as root bridges for different VLANs. The engineer wants to ensure that the access switch does not become the root bridge for any VLAN, even if the distribution switches fail. The engineer also wants to prevent any unauthorized switch from becoming root. What configuration should the engineer apply on the access switch?

Accepted Answer

Configure 'spanning-tree vlan 1-4094 priority 61440' and enable Root Guard on the uplink ports.. Option A is correct because setting the spanning-tree priority to 61440 (the highest possible value) ensures the access switch will never become the root bridge, even if the current root bridges fail. Enabling Root Guard on the uplink ports prevents any unauthorized switch from becoming root by placing the port into a root-inconsistent state if a superior BPDU is received, thus protecting the root bridge election.

Answer

Configure 'spanning-tree vlan 1-4094 priority 0' and enable BPDU Guard on the uplink ports.

Answer

Configure 'spanning-tree vlan 1-4094 priority 4096' and enable Loop Guard on the uplink ports.

Answer

Configure 'spanning-tree vlan 1-4094 priority 61440' and enable BPDU Guard on the uplink ports.

Question 5

A network engineer is troubleshooting a connectivity issue in a switched network. The network uses Rapid PVST+ with multiple VLANs. The engineer notices that a host connected to an access port on SW1 cannot communicate with the default gateway, which is on a distribution switch. The access port is configured with PortFast and BPDU Guard. The engineer checks the switch logs and sees that the port went into errdisable state. What is the most likely cause of the errdisable state?

Accepted Answer

Another switch was connected to the access port, causing BPDU Guard to disable the port.. The access port is configured with PortFast and BPDU Guard. PortFast immediately transitions the port to forwarding, but BPDU Guard monitors for incoming BPDUs. When another switch is connected to this access port, it sends BPDUs, triggering BPDU Guard to error-disable the port to prevent a potential bridging loop. This matches the log entry showing the port went into errdisable state.

Answer

A broadcast storm occurred due to a loop in the network.

Answer

The host connected to the port caused a duplex mismatch.

Answer

The cable connecting the host is faulty, causing link flaps.

Question 6

An engineer is designing a Layer 2 network with redundancy. The network uses MST (Multiple Spanning Tree) to reduce the number of STP instances. The engineer has configured two regions: Region 1 and Region 2. The engineer notices that switches in Region 1 are not forming a single MST region, and instead, they are treating each other as if they are in different regions. The engineer checks the configuration and finds that the region name and revision number are the same on all switches in Region 1, but the VLAN-to-instance mapping is different on one switch. What is the most likely cause of the issue?

Accepted Answer

The VLAN-to-instance mapping is not consistent across all switches in Region 1.. In MST, all switches within a region must agree on three parameters: the region name, the revision number, and the VLAN-to-instance mapping. Even if the region name and revision number match, a single mismatch in the VLAN-to-instance mapping causes the switches to treat each other as if they belong to different regions, preventing them from forming a single MST region.

Answer

The root bridge for each MST instance is not configured correctly.

Answer

BPDU Guard is enabled on the inter-switch links, preventing BPDU exchange.

Answer

PortFast is enabled on the inter-switch links, causing the switches to ignore BPDUs.

Question 7

A network engineer is troubleshooting an STP issue in a network that uses Rapid PVST+. The network has a root bridge (SW1) and a secondary root bridge (SW2). The engineer notices that after a link failure between SW1 and SW2, the network takes longer than expected to converge. The engineer checks the configuration and finds that SW2 has the 'spanning-tree uplinkfast' command enabled. The engineer also notices that SW2 has a lower priority than SW1. What is the most likely cause of the slow convergence?

Accepted Answer

UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence.. UplinkFast is a legacy STP feature that is incompatible with Rapid PVST+. When enabled on a switch running Rapid PVST+, it forces the switch to revert to 802.1D STP convergence behavior on the affected ports, disabling the rapid transition mechanisms (such as proposal/agreement and sync). This causes the network to take longer to converge after a link failure, as the switch falls back to the slower listening and learning states.

Answer

SW2 has a lower priority than SW1, so it takes longer to become the root bridge after failure.

Answer

BPDU Guard is enabled on the uplink ports, which prevents BPDU exchange.

Answer

Loop Guard is enabled on the uplink ports, which delays port transition.

Question 8

A network engineer is configuring a new switch that will be used as an access layer switch. The switch connects to two distribution switches via trunk links. The engineer wants to ensure that the access switch does not become the root bridge for any VLAN. The engineer also wants to provide redundancy so that if one uplink fails, the other uplink takes over quickly. The engineer is using Rapid PVST+. What configuration should the engineer apply on the access switch?

Accepted Answer

Configure 'spanning-tree vlan vlan-list priority 61440' on the access switch.. Option A is correct because setting the spanning-tree priority to 61440 (which is 0xF000 in hex) makes the switch a very unlikely root bridge candidate. In Rapid PVST+, the bridge priority is a 4-bit value (0-15) multiplied by 4096, so 61440 corresponds to priority 15 — the highest possible value. This ensures the access switch will never become the root bridge for any VLAN, while Rapid PVST+ provides fast failover (sub-second convergence) via its alternate/backup port mechanism without needing UplinkFast.

Answer

Configure 'spanning-tree vlan vlan-list priority 0' on the access switch.

Answer

Enable UplinkFast on the access switch to provide fast failover.

Answer

Enable PortFast on the trunk ports to speed up convergence.

Question 9

A network engineer is troubleshooting a Layer 2 loop that occurred in a network using Rapid PVST+. The network has three switches: SW1 (root), SW2, and SW3. The engineer examines the topology and finds that SW2 and SW3 are connected via a link that is not supposed to be there. The engineer suspects that an unauthorized switch was connected to the network, causing the loop. The engineer wants to prevent such loops in the future by configuring a feature that will disable any port that receives a BPDU from an unauthorized switch. Which feature should the engineer configure on the access ports?

Accepted Answer

Enable BPDU Guard on all access ports.. BPDU Guard is the correct feature because it immediately error-disables a port when a BPDU is received, preventing loops from unauthorized switches. Since the engineer wants to protect access ports from receiving BPDUs (which should never occur on a properly configured access port), BPDU Guard directly addresses the scenario of an unauthorized switch being connected and sending BPDUs.

Answer

Enable Loop Guard on all access ports.

Answer

Enable Root Guard on all access ports.

Answer

Enable UDLD on all access ports.

Question 10

A network engineer runs the following command on Switch SW1:

SW1# show spanning-tree vlan 10

VLAN0010
  Spanning tree enabled protocol ieee
  Root ID    Priority    24586
             Address     aabb.cc00.0100
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    24586  (priority 24576 sys-id-ext 10)
             Address     aabb.cc00.0100
             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               Desg FWD 4         128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Desg FWD 4         128.3    P2p

Based on this output, what can be concluded?

Accepted Answer

SW1 is the root bridge for VLAN 10, and all its interfaces are in the Designated role.. The output explicitly states 'This bridge is the root' and shows all three interfaces (Gi0/1, Gi0/2, Gi0/3) in the Designated (Desg) role with a Forwarding (FWD) state. In a stable STP topology, only the root bridge has all its active ports as Designated; non-root bridges will have at least one Root port. Therefore, SW1 is the root bridge for VLAN 10, and all its interfaces are in the Designated role.

Answer

SW1 is not the root bridge; the root bridge has MAC address aabb.cc00.0100 but is a different switch.

Answer

SW1 has a port in the Blocking state, preventing loops.

Answer

The root bridge priority for VLAN 10 is 24576 without the system ID extension.

Question 11

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?

Accepted Answer

GigabitEthernet0/3 is in the Blocking state to prevent a loop.. SW2 is not the root bridge (its bridge priority 32778 is higher than root priority 24596). Gi0/1 is the root port (cost 4 to root), Gi0/2 is a designated port, and Gi0/3 is an alternate port in blocking state. The root bridge is at MAC aabb.cc00.0200.

Answer

SW2 is the root bridge for VLAN 20.

Answer

The root port cost is 8.

Answer

SW2's bridge priority is 32768.

Question 12

A network engineer runs the following command on Switch SW3:

SW3# show spanning-tree vlan 30

VLAN0030
  Spanning tree enabled protocol ieee
  Root ID    Priority    24606
             Address     aabb.cc00.0400
             Cost        12
             Port        2 (GigabitEthernet0/2)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32798  (priority 32768 sys-id-ext 30)
             Address     aabb.cc00.0500
             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               Desg FWD 4         128.1    P2p
Gi0/2               Root FWD 12        128.2    P2p
Gi0/3               Desg FWD 4         128.3    P2p

Based on this output, what is the root path cost from SW3 to the root bridge for VLAN 30?

Accepted Answer

12. The root path cost is the cost of the path from this switch to the root bridge. It is shown in the Root ID section as 'Cost 12'. This is the cumulative cost via the root port (Gi0/2, which has a cost of 12).

Answer

4

Answer

16

Answer

20

Question 13

A network engineer runs the following command on Switch SW4:

SW4# show spanning-tree vlan 40

VLAN0040
  Spanning tree enabled protocol ieee
  Root ID    Priority    24616
             Address     aabb.cc00.0600
             Cost        8
             Port        1 (GigabitEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32768  (priority 32768 sys-id-ext 40)
             Address     aabb.cc00.0700
             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 8         128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Desg FWD 4         128.3    P2p

Based on this output, which port is the root port?

Accepted Answer

GigabitEthernet0/1. The root port is the port that provides the best path to the root bridge. In the output, Gi0/1 is listed as 'Root FWD', indicating it is the root port.

Answer

GigabitEthernet0/2

Answer

GigabitEthernet0/3

Answer

There is no root port because SW4 is the root bridge.

Question 14

A network engineer runs the following command on Switch SW5:

SW5# show spanning-tree vlan 50

VLAN0050
  Spanning tree enabled protocol ieee
  Root ID    Priority    24626
             Address     aabb.cc00.0800
             Cost        4
             Port        1 (GigabitEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32768  (priority 32768 sys-id-ext 50)
             Address     aabb.cc00.0900
             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               Desg FWD 4         128.3    P2p
Gi0/4               Altn BLK 4         128.4    P2p

Based on this output, how many ports are in the Forwarding state?

Accepted Answer

3. The output shows three ports in Forwarding state: Gi0/1 (Root FWD), Gi0/2 (Desg FWD), and Gi0/3 (Desg FWD). Gi0/4 is in Blocking state (Altn BLK).

Answer

1

Answer

2

Answer

4

Question 15

A network engineer runs the following command on Switch SW6:

SW6# show spanning-tree vlan 60

VLAN0060
  Spanning tree enabled protocol ieee
  Root ID    Priority    24636
             Address     aabb.cc00.0a00
             Cost        8
             Port        1 (GigabitEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32768  (priority 32768 sys-id-ext 60)
             Address     aabb.cc00.0b00
             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 8         128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Altn BLK 4         128.3    P2p
Gi0/4               Desg FWD 4         128.4    P2p

Based on this output, what is the bridge priority of the root bridge for VLAN 60?

Accepted Answer

24636. The root bridge priority is shown in the Root ID section as 24636. This includes the system ID extension of 60, so the base priority is 24576 (24636 - 60 = 24576).

Answer

24576

Answer

32768

Answer

32828

Question 16

A network engineer runs the following command on Switch SW7:

SW7# show spanning-tree vlan 70

VLAN0070
  Spanning tree enabled protocol ieee
  Root ID    Priority    24646
             Address     aabb.cc00.0c00
             Cost        4
             Port        1 (GigabitEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32768  (priority 32768 sys-id-ext 70)
             Address     aabb.cc00.0d00
             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, which port is the alternate port?

Accepted Answer

GigabitEthernet0/3. The alternate port is the port that provides an alternative path to the root bridge and is placed in blocking state. In the output, Gi0/3 is shown as 'Altn BLK'.

Answer

GigabitEthernet0/1

Answer

GigabitEthernet0/2

Answer

There is no alternate port.

Question 17

A network engineer runs the following command on Switch SW8:

SW8# show spanning-tree vlan 80

VLAN0080
  Spanning tree enabled protocol ieee
  Root ID    Priority    24656
             Address     aabb.cc00.0e00
             Cost        12
             Port        1 (GigabitEthernet0/1)
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    32768  (priority 32768 sys-id-ext 80)
             Address     aabb.cc00.0f00
             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 12        128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Desg FWD 4         128.3    P2p
Gi0/4               Altn BLK 4         128.4    P2p

Based on this output, what is the cost of the root port?

Accepted Answer

12. The root port is Gi0/1, and its cost is shown as 12 in the interface table.

Answer

4

Answer

8

Answer

16

Question 18

A network engineer runs the following command on Switch SW9:

SW9# show spanning-tree vlan 90

VLAN0090
  Spanning tree enabled protocol ieee
  Root ID    Priority    24666
             Address     aabb.cc00.1000
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

Bridge ID  Priority    24666  (priority 24576 sys-id-ext 90)
             Address     aabb.cc00.1000
             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               Desg FWD 4         128.1    P2p
Gi0/2               Desg FWD 4         128.2    P2p
Gi0/3               Desg FWD 4         128.3    P2p
Gi0/4               Desg FWD 4         128.4    P2p

Based on this output, what is the base priority of the root bridge?

Accepted Answer

24576. The root bridge priority is 24666, which includes the system ID extension of 90. The base priority is 24666 - 90 = 24576.

Answer

24666

Answer

32768

Answer

32858

Question 19

interface GigabitEthernet0/1
 spanning-tree portfast
 spanning-tree bpduguard enable
 end

What is the effect of this configuration?

Accepted Answer

The port will immediately transition to forwarding state and will be error-disabled if a BPDU is received.. PortFast and BPDU Guard are enabled on the interface, which immediately transitions the port to forwarding and disables it if a BPDU is received.

Answer

The port will go through normal STP states and will be error-disabled if a BPDU is received.

Answer

The port will immediately transition to forwarding and ignore any BPDUs received.

Answer

The port will remain in blocking state until a BPDU is received.

Question 20

spanning-tree vlan 10 priority 4096

What is the effect of this global configuration command?

Accepted Answer

The switch will have a bridge priority of 4096 for VLAN 10, increasing its chance to become root bridge.. This sets the bridge priority for VLAN 10 to 4096, making the switch more likely to become the root bridge for that VLAN.

Answer

The switch will have a bridge priority of 4096 for all VLANs.

Answer

The switch will become the root bridge for VLAN 10 immediately.

Answer

The switch will have a bridge priority of 4096 for VLAN 10 and all other VLANs will use 32768.