CCNP Practice Question: Is troubleshooting an STP issue in a switched…

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.