Which STP port type is permitted to forward traffic, but is not the port closest to the root bridge?

Question

BlackTom AI · Accepted Answer

A designated port is the port on a LAN segment chosen to forward frames out of that segment and therefore is placed in the forwarding state, but it is not the port on a switch that is closest to the root bridge.

1) Relevant concepts:
- Spanning Tree Protocol (STP) assigns port roles to prevent loops: root port, designated port, and non-designated (blocking) ports. Rapid STP adds alternative and backup roles.
- Root port (RP): on each non-root switch, the single port with the lowest path cost to the root bridge; this port forwards toward the root.
- Designated port (DP): on each network segment, the port selected as having the best path to the root; it forwards frames for that segment.
- Alternative/backup ports: redundant ports that are placed in a non-forwarding (discarding/blocking) state until needed.

2) Step-by-step reasoning:
- The question asks for a port type that is permitted to forward traffic but is not the port closest to the root bridge.
- By definition, the root port is the port closest to the root bridge on a given switch — it is therefore the port “closest to the root,” so it does not meet the “not closest” requirement.
- A designated port is chosen per LAN segment and is placed in the forwarding state to send traffic onto that segment. The designated port is not the same as the root port (unless that same physical port happens to also be the root port on a specific switch); generally, the designated port is not defined by being the closest to the root bridge.
- Alternative and backup ports are redundant ports kept in a non-forwarding state; they are not permitted to forward under normal conditions.

3) Intermediate/definitional steps:
- Root port = closest-path-to-root on a switch → forwards.
- Designated port = best-port-for-a-segment → forwards.
- Alternative/Backup = redundant → blocked (not forwarding).

4) Why the correct answer is correct:
- Designated ports are explicitly permitted to forward traffic for their segment, and they are distinct from root ports (which are the closest to the root bridge). Thus they satisfy both parts of the question.

5) Why other options are incorrect:
- Root port: is the port closest to the root bridge (contradicts the “not closest” condition).
- Alternative port: in RSTP is a backup/alternate path and is in a non-forwarding state.
- Backup port: provides redundancy on the same segment and is also non-forwarding in normal operation.

Conclusion: The correct answer is designated port, which matches the provided answer.

Which STP port type is permitted to forward traffic, but is not the port closest to the root bridge?单项选择题

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