New CCNA – STP Questions & Answers with Explanations

By | September 9, 2017

This is an Updated CCNA exam question ( 2016). You might see a different IP addressing, VLAN configuration and Port allocation .As usual, take time to read through the question so as to clearly understand what the question is about.

Note: This STP questions are used for demonstration purposes only. You might see different IP addressing,  configuration and Port allocation in the real CCNA exam. But it all works the same way, try and understand the technique.

Read STP Explained

I suggest you use packet tracer for practice.

Question 1
Which three statements accurately describe Layer 2 Ethernet switches? (Choose three.)
A. Spanning Tree Protocol allows switches to automatically share VLAN information.
B. Establishing VLANs increases the number of broadcast domains.
C. Switches that are configured with VLANs make forwarding decisions based on both Layer 2 and Layer 3 address information.
D. Microsegmentation decreases the number of collisions on the network.
E. In a properly functioning network with redundant switched paths, each switched segment will contain one root bridge with all its ports in the forwarding state. All other switches in that broadcast domain will have only one root port.
F. If a switch receives a frame for an unknown destination, it uses ARP to resolve the address.

Answer : B,D,E

Micro-segmentation is a network design (functionality) where each workstation or device on a network gets its own dedicated segment (collision domain) to the switch.

 Each network device gets the full bandwidth of the segment and does not have to share the segment with other devices. Micro-segmentation reduces and can even eliminate collisions because each segment is its own collision domain.

Question 2
Refer to the exhibit.

A technician has installed SwitchB and needs to configure it for remote access from the management workstation. What command line would enable this?

A. SwitchB(config)# interface FastEthernet 0/1
SwitchB(config-if)# ip address

SwitchB(config-if)# no shutdown

B. SwitchB(config)# interface vlan 1
SwitchB(config-if)# ip address
SwitchB(config-if)# ip default-gateway                                 SwitchB(config-if)# no shutdown
C. SwitchB(config)# ip default-gateway
SwitchB(config)# interface vlan 1
SwitchB(config-if)# ip address
SwitchB(config-if)# no shutdown
D. SwitchB(config)# ip default-network
SwitchB(config)# interface vlan 1
SwitchB(config-if)# ip address
SwitchB(config-if)# no shutdown
E. SwitchB(config)# ip route
SwitchB(config)# interface FastEthernet 0/1
SwitchB(config-if)# ip address
SwitchB(config-if)# no shutdown

Answer: C
To remote access to SwitchB, it must have a management IP address on a VLAN on that switch. normally, we often use VLAN 1 as the management.

 In the exhibit, we can see that the Management Workstation is in a different subnet from the SwitchB. For communication to occur between the subnetwork, you must configure at least one default gateway.

This default gateway is used to forward traffic originating from the switch only, not to forward traffic sent by devices connected to the switch.

Question 3

A switch is configured with all ports assigned to VLAN 2 with full duplex FastEthernet to segment existing departmental traffic. What is the effect of adding switch ports to a new VLAN on the switch?
A. More collision domains will be created.
B. IP address utilization will be more efficient.
C. More bandwidth will be required than was needed previously.
D. An additional broadcast domain will be created.

Answer : D

Each VLAN creates its own broadcast domain. Since this is a full duplex switch, each port is a separate collision domain.

Question 4
Which command enables RSTP on a switch?
A. spanning-tree uplinkfast
B. spanning-tree mode rapid-pvst
C. spanning-tree backbonefast
D. spanning-tree mode mst

Answer : B

Rapid Spanning Tree Protocol (RSTP) is an enhancement of the original STP 802.1D protocol. The RSTP 802.1w protocol is an IEEE open implementation. Cisco has its own proprietary implementation of RSTP, that includes the benefits of its Per-VLAN spanning tree protocols, called Rapid-PVST+. To activate the Rapid-PVST+ protocol use the command:
switch(config)#spanning-tree mode rapid-pvst

Question 5
Refer to the exhibit.

Which switch provides the spanning-tree designated port role for the network segment that services the printers?

A. Switch1
B. Switch2
C. Switch3
D. Switch4

Answer : C

Printers are connected by hubs. Decide the switch that provides the spanning-tree designated port role between Switch3 and Switch4. They have the same priority 32768. Compare their MAC addresses. Switch3 with a smaller MAC address will provide a designated port for printers.

Question 6
Which switch would STP choose to become the root bridge in the selection process?
A. 32768: 11-22-33-44-55-66
B. 32768: 22-33-44-55-66-77
C. 32769: 11-22-33-44-55-65
D. 32769: 22-33-44-55-66-78

Answer : A
The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a configurable priority number and a MAC Address; the bridge ID contains both numbers combined together – Bridge priority + MAC (32768.0200.0000.1111).

The Bridge priority default is 32768 and can only be configured in multiples of 4096(Spanning tree uses the 12 bits extended system ID). To compare two bridge IDs, the priority is compared first, as if looking at a real number anything less than 32768…will become the target of being the root.

If two bridges have equal priority then the MAC addresses are compared; for example, if switches A (MAC=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 32768 then switch A will be selected as the root bridge. In this case, 32768: 11-22-33-44-55-66 would be the bridge because it has a lower priority and MAC address.

Question 7
Refer to the exhibit.

Why has this switch not been elected the root bridge for VLAN1?

A. It has more than one interface that is connected to the root network segment.
B. It is running RSTP while the elected root bridge is running 802.1d spanning tree.
C. It has a higher MAC address than the elected root bridge.
D. It has a higher bridge ID than the elected root bridge.

Answer : D

The root bridge is determined by the lowest bridge ID, and this switch has a bridge ID priority of 32768, which is higher than the roots priority of 20481.

Question 8

Refer to the exhibit.

The output that is shown is generated at a switch. Which three statements are true?(Choose three.)

A. All ports will be in a state of discarding, learning, or forwarding.
B. Thirty VLANs have been configured on this switch.
C. The bridge priority is lower than the default value for spanning tree.
D. All interfaces that are shown are on shared media.
E. All designated ports are in a forwarding state.
F. This switch must be the root bridge for all VLANs on this switch.
Answer : A,C,E
From the output above, we see that all ports are in a designated role (forwarding state). The command show spanning-tree vlan 30 only shows us information about VLAN 30. It’s not known how many VLAN exists in this switch.

The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768. All three interfaces on this switch have the connection type p2p, which means Point-to-point environment, not a shared media. From this we can specify is this switch is the root bridge for VLAN 30 but there is no guarantee it is also the root bridge for other VLANs.

Question 9
Refer to the topology shown in the exhibit. Which ports will be STP designated ports if all the links are operating at the same bandwidth? (Choose three)

A. Switch A – Fa0/0
B. Switch A – Fa0/1
C. Switch B – Fa0/0
D. Switch B – Fa0/1
E. Switch C – Fa0/0
F. Switch C – Fa0/1

Answer: B C D
Comparing the shown MAC addresses we can say that switch B will be root bridge as it has lowest MAC. That means all of its ports are designated ports; with that,  C & D are correct.

When you look at the link between switch A & switch C there must be at least one designated port and one non-designated (blocked) port. To figure out which port is designated port , we compare their MAC address again. A has lower MAC so Fa0/1 of switch A will be designated port while Fa0/1 of switch C will be blocked, B is is the best option as an answer.

Question 10
What value is primarily used to determine which port becomes the root port on each non-root switch in a spanning-tree topology?
A. lowest port MAC address
B. port priority number and MAC address.
C. VTP revision number
D. highest port priority number.
E. path cost

Answer: E
The path cost to the root bridge is the most important value to determine which port will become the root port on each non-root switch. In particular, the port with the lowest cost to the root bridge will become root port (on non-root switch).

Question 11
What is one benefit of PVST+?
A. PVST+ reduces the CPU cycles for all the switches in the network.
B. PVST+ automatically selects the root bridge location, to provide optimization.
C. PVST+ allows the root switch location to be optimized per vlan.
D. PVST+ supports Layer 3 load balancing without loops.

Answer: C
Per VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It means a switch can be the root bridge of a VLAN while another switch can be the root bridge of other VLANs in a common topology.

For example, Switch 1 can be the root bridge for Voice data while Switch 2 can be the root bridge for Video data. If designed correctly, it can optimize the network traffic.

Question 12
Which two protocols are used by bridges and/or switches to prevent loops in a layer 2 network? (Choose two)
A. 802.1d
C. 802.1q

Answer: A D
Question 13
In which circumstance are multiple copies of the same unicast frame likely to be transmitted in a switched LAN?A. after broken links are re-established
B. in an improperly implemented redundant topology
C. when upper-layer protocols require high reliability
D. during high traffic periods
E. when a dual ring topology is in use

Answer: B
If we connect two switches via 2 or more links and do not enable STP on these switches then a loop (which creates multiple copies of the same unicast frame) will occur. It is an example of an improperly implemented redundant topology.
A. Host B would not be able to access the server in VLAN9 until the cable is reconnected.
B. Communication between VLAN3 and the other VLANs would be disabled.
C. The transfer of files from Host B to the server in VLAN9 would be significantly slower.
D. For less than a minute, Host B would not be able to access the server in VLAN9. Then normal network function would resume.

Question 14
Which port state is introduced by Rapid-PVST?A. learning
B. listening
C. discarding
D. forwarding

Answer: C
PVST+ is based on IEEE802.1D Spanning Tree Protocol (STP). In saying that,  PVST+ has only 3 port states (discarding, learning and forwarding) while STP has 5 port states (blocking, listening, learning, forwarding and disabled). So discarding is a new port state in PVST+.
Question 15

Refer to the exhibit.

Based on the information given, which switch will be elected root bridge and why? A. Switch A, because it has the lowest MAC address
B. Switch A, because it is the most centrally located switch
C. Switch B, because it has the highest MAC address
D. Switch C, because it is the most centrally located switch
E. Switch C, because it has the lowest priority
F. Switch D, because it has the highest priority

Answer: E

Question 16
Refer to the exhibit.

Given the output is shown from this Cisco Catalyst 2950, what is the most likely reason that interface FastEthernet 0/10 is not the root port for VLAN 2?

Switch# show spanning-tree interface fastethernet0/10

A. This switch has more than one interface connected to the root network segment in VLAN 2.
B. This switch is running RSTP while the elected designated switch is running 802.1d Spanning Tree.
C. This switch interface has a higher path cost to the root bridge than another in the topology.

D. This switch has a lower bridge ID for VLAN 2 than the elected designated switch.

Answer: C

Question 17
Which term describes a spanning-tree network that has all switch ports in either theblocking or forwarding state?

A. converged
B. redundant
C. provisioned
D. spanned

Answer : A

Spanning Tree Protocol convergence (Layer 2 convergence) happens when bridges and switches have transitioned to either the forwarding or blocking state. When layer 2 is converged, root bridge is elected and all port roles (Root, Designated and Non-Designated) in all switches are selected.