# CCNA 200-125 Exam: STP Questions With Answers

1. 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

Show (Hide) Explanation/Reference
First by comparing their MAC addresses we learn that switch B will be root bridge as it has lowest MAC. Therefore all of its ports are designated ports -> C & D are correct.

On the link between switch A & switch C there must have one designated port and one non-designated (blocked) port. We can figure out which port is designated port by comparing 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 correct.

2. If the primary root bridge experiences a power loss, which switch takes over?
• A. switch 0040.0ВС0.90C5
• B. switch 00E0.F90B.6BE3
• C. switch 0004.9A1A.C182*
• D. switch 00E0.F726.3DC6

Show (Hide) Explanation/Reference
The switches compare their Bridge ID with each other to find out who will be the root bridge. The root bridge is the bridge with the lowest bridge ID.

Bridge ID = Bridge Priority + MAC Address

In this question the bridge priority was not mentioned so we suppose they are the same. Therefore the switch with lowest MAC address will become the new root bridge.

3. If primary and secondary root switches with priority 16384 both experience catastrophic losses, which tertiary switch can take over?
• A. a switch with priority 20480*
• B. a switch with priority 8192
• C. a switch with priority 4096
• D. a switch with priority 12288

Show (Hide) Explanation/Reference
This is a tricky question. We know the switch with lowest value of priority is elected the root switch. Therefore in this question the switches with priority of 4096, 8192, 12288 (which are lower than the current value of the root bridge 16384) are not joining the root bridge election by somehow. The only suitable answer is the switch with priority 20480 will become the root bridge.
4. Which spanning-tree protocol rides on top of another spanning-tree protocol?
• A. MSTP*
• B. RSTP
• C. PVST+
• D. Mono Spanning Tree

Show (Hide) Explanation/Reference
Multiple Spanning Tree (MST) rides on top of RSTP so it converges very fast. The idea behind MST is that some VLANs can be mapped to a single spanning tree instance because most networks do not need more than a few logical topologies.
5. Which IEEE standard does PVST+ use to tunnel information?
• A. 802.1x
• B. 802.1q*
• C. 802.1w
• D. 802.1s

Show (Hide) Explanation/Reference
Cisco developed PVST+ to allow strolling numerous STP instances, even over an 802.1Q network via the use of a tunneling mechanism. PVST+ utilizes Cisco gadgets to hook up with a Mono Spanning Tree area to a PVST+ region. No particular configuration is needed to attain this. PVST+ affords assist for 802.1Q trunks and the mapping of a couple of spanning trees to the single spanning tree of popular 802.1Q switches strolling Mono Spanning Tree.
6. Which process is associated with spanning-tree convergence?
• A. determining the path cost
• B. electing designated ports*
• C. learning the sender bridge ID
• D. assigning the port ID

Show (Hide) Explanation/Reference
SPT must performs three steps to provide a loop-free network topology:

1. Elects one root bridge
2. Select one root port per nonroot bridge
3. Select one designated port on each network segment -> Answer B is correct.

7. Refer to the exhibit. The output that is shown is generated at a switch. Which three of these 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. The switch must be the root bridge for all VLANs on this switch.

Show (Hide) Explanation/Reference
From the output, we see that all ports are in Designated role (forwarding state) -> A and E are correct.

The command “show spanning-tree vlan 30″ only shows us information about VLAN 30. We don’t know how many VLAN exists in this switch -> B is not correct.

The bridge priority of this switch is 24606 which is lower than the default value bridge priority 32768 -> C is correct.

All three interfaces on this switch have the connection type “p2p”, which means Point-to-point environment – not a shared media -> D is not correct.

The only thing we can specify is this switch is the root bridge for VLAN 30 but we can not guarantee it is also the root bridge for other VLANs -> F is not correct.

8. When an interface is configured with PortFast BPDU guard, how does the interface respond when it receives a BPDU?
• A. It continues operating normally.
• B. It goes into an errdisable state.*
• C. It goes into a down/down state.
• D. It becomes the root bridge for the configured VLAN.

Show (Hide) Explanation/Reference
PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state.

In a valid configuration, PortFast-configured interfaces do not receive BPDUs (because PortFast should only be configured on interfaces which are connected to hosts). If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.