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Difficulty: MediumSpanning Tree Protocol Operations

Three switches—Switch A, Switch B, and Switch C—are interconnected in a redundant Layer 2 topology running standard IEEE 802.1D Spanning Tree Protocol (STP). Switch A is configured with a bridge priority of 4096, while Switch B and Switch C both retain the default bridge priority of 32768. The link between Switch A and Switch B is 1 Gbps (cost = 4), the link between Switch A and Switch C is 10 Gbps (cost = 2), and the link between Switch B and Switch C is 1 Gbps (cost = 4). Based on this topology and STP operational rules, which of the following statements correctly describe the resulting converged network state?

  1. Switch A is elected as the Root Bridge for the topology because it has the lowest bridge priority value.Answer
  2. Switch C's interface connected directly to Switch A is selected as Switch C's Root Port due to having the lowest path cost to the Root Bridge.Answer
  3. C
    Switch B elects its interface connected to Switch C as a Designated Port because Switch B has a lower MAC address than Switch C.
  4. D
    Switch B can force itself to become the Root Bridge by increasing its bridge priority setting to a higher numerical value.
  5. E
    A native VLAN mismatch on the trunk between Switch B and Switch C will force Switch C to automatically become a secondary Root Bridge.

Answer

Switch A is elected as the Root Bridge for the topology because it has the lowest bridge priority value, and Switch C's interface connected directly to Switch A is selected as Switch C's Root Port due to having the lowest path cost to the Root Bridge.
Switch A becomes the Root Bridge because its priority of 4096 is lower than all other switches. Switch C selects its direct 10 Gbps interface (cost 2) as its Root Port because it provides the lowest cumulative path cost to the Root Bridge compared to any alternate route.

Step-by-Step Solution

1
Determine the Root Bridge election result by comparing Bridge IDs (Bridge Priority + MAC Address).
Switch A has priority 4096, which is lower than Switch B (32768) and Switch C (32768). Switch A becomes the Root Bridge.
STP selects the switch with the lowest numerical Bridge ID as the Root Bridge.
2
Determine Root Ports for non-root switches by evaluating cumulative root path costs.
Switch C's direct link to Switch A has a cost of 2 (10 Gbps link), which is lower than the indirect path through Switch B (cost 8). The direct port is chosen as Switch C's Root Port.
Each non-root switch chooses the single port with the lowest path cost back to the Root Bridge.
3
Evaluate the port roles on the link between Switch B and Switch C.
Switch C advertises a cost of 2 to reach the root, while Switch B advertises a cost of 4. Switch C wins designated port status on that link, placing Switch B's port on that segment in the Blocking state.
Designated Ports on shared segments are assigned to the switch with the lowest root path cost.

Key Concept

Spanning Tree Protocol Operations (Root Bridge Election, Path Cost Calculation, and Port Role Determination)
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Topics

Spanning Tree Protocol Operations

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