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Three switches, SW1, SW2 and SW3, are connected in a triangle with one link between each pair. The STP root bridge is SW1, and SW2 has a lower bridge ID than SW3. All links have the same path cost. Which port is blocked (does not forward frames)?

  1. AThe port on SW1 that connects to SW3
  2. BThe port on SW2 that connects to SW3
  3. CThe port on SW3 that connects to SW2
Show answer and explanation

Answer: C. The port on SW3 that connects to SW2

All ports on the root bridge are designated ports in the forwarding state. SW2 and SW3 each choose as their root port the port with the lowest cost to the root bridge, which is the port connected directly to SW1. On the remaining SW2–SW3 link, both switches have the same cost to the root bridge (one link), so the port on SW2, which has the lower bridge ID, becomes the designated port. The port on SW3 is neither a root port nor a designated port, so it is blocked and the triangular loop is broken.

  • A. Ports on the root bridge are always designated ports and forward frames; they are never blocked.
  • B. SW2 has the lower bridge ID, so its port is the designated port on this link and forwards frames.
  • C. Correct. It is neither a root port nor a designated port, so it is blocked.
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