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Difficulty: MediumSpanning Tree Protocol Operations

In a switched network running standard IEEE 802.1D Spanning Tree Protocol (STP), Switch-C is connected to the Root Bridge (Switch-A) via a direct 100 Mbps link with a path cost of 19. Switch-C is also connected to Switch-B via a 1 Gbps link with a path cost of 4. Switch-B connects directly to Switch-A via a 1 Gbps link with a path cost of 4. Which port will Switch-C select as its Root Port, and what is the calculated Root Path Cost for that selection?

  1. The port connected to Switch-B, with a calculated Root Path Cost of 8.Answer
  2. B
    The port connected to Switch-A, with a calculated Root Path Cost of 19.
  3. C
    The port connected to Switch-B, with a calculated Root Path Cost of 4.
  4. D
    The port connected to Switch-A, because direct links bypass root path cost evaluations.

Answer

The port connected to Switch-B, with a calculated Root Path Cost of 8.
Each non-root switch selects exactly one Root Port, which is the port providing the lowest cumulative path cost to the Root Bridge. The path via Switch-B totals a cost of 4 + 4 = 8, whereas the direct link to Switch-A has a cost of 19. Therefore, Switch-C designates the port connected to Switch-B as its Root Port with a Root Path Cost of 8.

Step-by-Step Solution

1
Determine the Root Path Cost for the direct connection from Switch-C to Switch-A.
Direct path cost = 19 (cost of the 100 Mbps link).
The cost advertised by the Root Bridge is 0, and Switch-C adds the port cost of 19.
2
Determine the Root Path Cost for the indirect connection from Switch-C through Switch-B to Switch-A.
Indirect path cost = 4 + 4 = 8.
Switch-B receives BPDU from Root Bridge with cost 0 and adds its link cost of 4 (advertising cost 4 to Switch-C). Switch-C then adds its local link cost of 4 to Switch-B.
3
Compare the path costs to select the Root Port.
8 < 19, so Switch-C selects the port leading to Switch-B as its Root Port.
STP selects the Root Port based on the lowest cumulative Root Path Cost.

Key Concept

STP Root Port Selection based on cumulative Root Path Cost
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Topics

Spanning Tree Protocol Operations

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