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350-401 Practice Question: Is troubleshooting an STP issue in a switched…
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?
⚠ Common exam trap
Cisco often tests the misconception that increasing a switch's priority (making it numerically higher) helps it become root, when in fact the root bridge is elected based on the lowest bridge priority value.
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 the 'spanning-tree vlan vlan-id root primary' command on the intended distribution switch.
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 analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Configure the 'spanning-tree vlan vlan-id root primary' command on the intended distribution switch.
Why this is correct
The `spanning-tree vlan vlan-id root primary` command is a Cisco macro that forces the switch to become the root bridge by automatically setting its bridge priority to 24576, or to 4096 less than the current lowest priority if another switch already has a priority below 24576. This adjusts the priority dynamically without manual calculation, ensuring the intended distribution switch wins the root election. The command also configures the switch to be the primary root for that VLAN, which is exactly the desired outcome.
- ✗
Set the priority of the access switch to 0 using the 'spanning-tree vlan vlan-id priority 0' command.
Why it's wrong here
Setting the access switch's priority to 0 using `spanning-tree vlan vlan-id priority 0` would make it the root bridge because 0 is the lowest possible bridge priority, giving it precedence over all other switches in the VLAN. This is incorrect because the engineer wants the distribution switch to be the root, not the access switch. Additionally, forcing an access-layer switch to be root can cause suboptimal traffic forwarding, as all inter-VLAN traffic would need to traverse the access layer, which is not designed for such a role.
- ✗
Increase the priority of the distribution switch to 61440 using the 'spanning-tree vlan vlan-id priority 61440' command.
Why it's wrong here
Increasing the distribution switch's priority to 61440 via `spanning-tree vlan vlan-id priority 61440` would make it less likely to become the root bridge, because a higher numeric priority value indicates a lower priority in STP. Since the root bridge is elected as the switch with the lowest priority (and bridge ID), setting the distribution switch to 61440 would actually push it toward the edge of the spanning tree topology. The engineer's goal is to make the distribution switch the root, so this command directly contradicts the intent and could result in an unexpected access switch becoming root.
- ✗
Disable STP on the distribution switch and manually configure it as the root bridge.
Why it's wrong here
Disabling STP on the distribution switch removes its ability to participate in the dynamic root bridge election, but it does not manually configure it as the root bridge; a root bridge must still be elected through BPDUs, and without STP running, the switch will not advertise its bridge ID. Furthermore, disabling STP creates a serious risk of Layer 2 loops, which can cause broadcast storms, MAC address flapping, and network outages. Even if you could manually 'force' a root, this approach is unsafe and violates best practices; the correct way is to use STP commands that adjust the bridge priority or use the root primary macro.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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