Refer to the exhibit. In the Core-2 configuration of spanning-tree instance 2 priority 0, what needs to be configured to enable the root for VLAN 20 while VLAN 10 remains root on Core-1?

Question

Options

• ASpanning-tree instance 2 VLAN 20
• BSpanning-tree priority 0 VLAN 20
• CSpanning-tree priority root VLAN 20
• DSpanning-tree VLAN 20

• A
  83% (45)
• B
  11% (6)
• C
  4% (2)
• D
  2% (1)

Explanation

Option A is correct because in Multiple Spanning Tree (MST), VLANs must be explicitly mapped to instances. Since Core-2 already has spanning-tree instance 2 priority 0 configured - making it the root bridge for instance 2 - adding spanning-tree instance 2 VLAN 20 maps VLAN 20 into that instance, so Core-2 becomes root for VLAN 20 without touching VLAN 10's instance on Core-1.

Why the distractors fail:

• B (priority 0 VLAN 20) - invalid syntax; priority is set at the instance level, not per-VLAN in MST, and the argument order is wrong.
• C (priority root VLAN 20) - priority root is not valid MST syntax; root primary is a PVST+ shortcut and doesn't belong in an MST context.
• D (spanning-tree VLAN 20) - an incomplete, non-functional command that configures nothing meaningful.

Memory tip: Think of MST as a two-layer system - instances carry priority, VLANs ride instances. If the priority is already set on an instance, your only remaining job is to assign the VLAN to that instance. Always ask: "Is the VLAN mapped to the right instance?"

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