CCNA Switching and Network Access Practice Question
Network Topology
You are connected to a multilayer switch MLS1. The network has two other switches: SW2 and SW3. The interface GigabitEthernet0/1 already has PortFast and BPDU Guard enabled. Configure MLS1 as the root bridge for VLAN 10 and VLAN 20 using the root primary command. After configuration, verify that the interface is not in err-disabled state and that the root bridge role is correctly assigned.
⚠ Common exam trap
Do not confuse 'root primary' with 'root secondary' or manual priority settings. The 'root primary' command automatically sets the priority to 24576, which is the recommended value. Also, avoid adding unnecessary global commands when the interface already has the required features configured.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.
The interface Gi0/1 already has PortFast and BPDU Guard configured, so no additional configuration is needed for that step. Using 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' sets the priority to 24576, ensuring MLS1 becomes root for both VLANs. Verify with 'show spanning-tree vlan 10' to see the priority changed and 'show interfaces gigabitEthernet0/1 status' to confirm the port is not err-disabled.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.
Why this is correct
This is correct because 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' are the standard Cisco IOS macros that force MLS1 to be the root bridge for both VLANs, automatically setting its priority to 24576, or to 4096 if a lower-priority bridge already exists. The verification commands are appropriate: 'show spanning-tree vlan 10' displays the Root ID field, which must match MLS1's bridge ID for VLAN 10, and 'show interfaces gigabitEthernet0/1 status' confirms the interface is up/up and not error-disabled, which is essential since PortFast and BPDU Guard are already enabled on that port. This combination completes the required root configuration and properly validates both the spanning-tree election and the health of the access port.
- ✗
Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root secondary' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.
Why it's wrong here
This configuration is incorrect for VLAN 20 because 'spanning-tree vlan 20 root secondary' sets MLS1's bridge priority to 28672, which is the backup-root value, not the primary-root value of 24576 used by 'root primary'. While MLS1 would still outbid a switch at the default priority 32768, the command explicitly designates it as the standby root, contrary to the requirement that MLS1 be the root for both VLANs. Moreover, the verification commands only inspect VLAN 10 via 'show spanning-tree vlan 10' and the interface status, leaving VLAN 20's root election completely unverified.
- ✗
Configure 'spanning-tree vlan 10 priority 4096' and 'spanning-tree vlan 20 priority 4096' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.
Why it's wrong here
This is incorrect because setting the priority to 4096 is not the standard method; the 'root primary' command automatically sets the priority to 24576 (or lower if needed). A priority of 4096 is lower than 24576 and would also make the switch root, but the question specifically asks to use the 'root primary' command.
- ✗
Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' on MLS1. Then configure 'spanning-tree portfast default' and 'spanning-tree bpduguard default' globally. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.
Why it's wrong here
This is incorrect because the question states that PortFast and BPDU Guard are already enabled on G0/1. Configuring them globally would apply to all access ports, which is unnecessary and could cause issues on trunk ports. The existing configuration on the interface is sufficient.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.Correct answer▾
Why this is correct
This is correct because 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' are the standard Cisco IOS macros that force MLS1 to be the root bridge for both VLANs, automatically setting its priority to 24576, or to 4096 if a lower-priority bridge already exists. The verification commands are appropriate: 'show spanning-tree vlan 10' displays the Root ID field, which must match MLS1's bridge ID for VLAN 10, and 'show interfaces gigabitEthernet0/1 status' confirms the interface is up/up and not error-disabled, which is essential since PortFast and BPDU Guard are already enabled on that port. This combination completes the required root configuration and properly validates both the spanning-tree election and the health of the access port.
✗Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root secondary' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.Wrong answer — click to see why▾
Why this is wrong here
Using root secondary for VLAN 20 would not make MLS1 root for that VLAN; it would set a higher priority, leaving another switch as root.
Why candidates choose this
Candidates may confuse 'root primary' and 'root secondary', thinking both are needed for redundancy, but the requirement is for MLS1 to be the root, not a backup.
✗Configure 'spanning-tree vlan 10 priority 4096' and 'spanning-tree vlan 20 priority 4096' on MLS1. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.Wrong answer — click to see why▾
Why this is wrong here
While a priority of 4096 would make MLS1 root, the 'root primary' command is the recommended method and automatically sets the appropriate value (24576).
Why candidates choose this
Candidates might think manually setting a low priority is equivalent to 'root primary', but the command is a best practice and ensures proper operation with other switches.
✗Configure 'spanning-tree vlan 10 root primary' and 'spanning-tree vlan 20 root primary' on MLS1. Then configure 'spanning-tree portfast default' and 'spanning-tree bpduguard default' globally. Verify with 'show spanning-tree vlan 10' and 'show interfaces gigabitEthernet0/1 status'.Wrong answer — click to see why▾
Why this is wrong here
PortFast and BPDU Guard are already enabled on the interface; adding global defaults is unnecessary and does not complete the specific interface task, though it would not harm the root bridge goal.
Why candidates choose this
Candidates might think global configuration is a best practice, but the question specifies the interface already has these features enabled, so additional configuration is unnecessary and could be counterproductive.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Key term
Root bridge
The root bridge is the central reference point in a Spanning Tree Protocol (STP) network, serving as the logical root of the spanning tree topology.
Key term
Bridge Protocol Data Unit
A Bridge Protocol Data Unit (BPDU) is a special message that network switches exchange to detect and prevent loops in an Ethernet network.
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This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.