In Spanning Tree Protocol (STP), traffic forwarding follows the loop-free topology established. To determine the path from the root bridge (SW1) to any other switch (SW4), we must first determine the lowest cost path from SW4 to the root bridge. This path's ingress port on SW4 becomes its Root Port.

The possible path costs from SW4 to SW1 are:

1. SW4 → SW3 → SW1: Cost = 5 + 10 = 15

2. SW4 → SW2 → SW1: Cost = 10 + 10 = 20

3. SW4 → SW1 (direct): Cost = 25

The path via SW3 has the lowest cost (15). Therefore, traffic from SW1 to SW4 will follow the reverse path: SW1 → SW3 → SW4.

A. The path via SW2 has a higher root path cost (20) compared to the path via SW3 (15), so it is not selected.

C. This path is suboptimal and has an even higher cost (25). STP would not select this redundant path.

D. The direct link has the highest cost (25) and will be blocked on the SW4 side to prevent a loop.

1. Huawei

HCIA-Datacom V1.0 Training Material (2020)

"STP/RSTP Implementation

" Section: STP Path Cost

pp. 198-199. The material explains that a switch selects the path with the lowest root path cost to reach the root bridge.

2. Huawei

HedEx Documentation for VRP V800R012C10

"STP/RSTP Configuration

" Section: STP Algorithm. This section details how the root path cost is calculated by summing the costs of all links on the path to the root bridge.