Which port role will be assigned to all active trunk switch ports on the root bridge of a Layer 2 switched domain?

Question

BlackTom AI · Accepted Answer

All active trunk switch ports on the root bridge become designated ports because the root bridge has the best (zero) path to itself on every connected LAN segment, so its ports are selected as the forwarding ports for those segments.

1. Relevant knowledge points and concepts
- Spanning Tree Protocol (STP) elects a single root bridge for a Layer 2 domain.  
- STP assigns port roles: root port (on non-root switches, the port with the lowest path cost toward the root), designated port (the port on a LAN segment that has the lowest path cost to the root and forwards frames for that segment), alternate port (a blocked port providing a loop-free backup), and disabled/administratively down.  
- Path cost to the root from the root bridge itself is 0.

2. Step-by-step reasoning process
- On any given broadcast/multi-access segment, STP needs one forwarding port that represents the best path to the root; that is the designated port for that segment.  
- The root bridge, by definition being the root, has path cost 0 to itself. For every segment where the root bridge is connected, the root bridge’s connected port will have the lowest path cost for that segment (0), so that port is chosen as the designated port.  
- Other switches connected to that same segment will compare their path costs to the root. Since their costs are greater than 0, they cannot be designated for that segment; one may be assigned a root port (if it uses that segment as its chosen path to the root) or an alternate/blocked port (if it would create a loop).

3. Intermediate logic/calculations
- Path cost(root bridge port) = 0.  
- Other switch path cost > 0.  
- Lowest path cost wins → root bridge port is designated.

4. Why the correct answer is correct
Because STP selects the port with the lowest path cost to the root as the designated port on each segment, and the root bridge’s ports have the absolute lowest cost (zero), all active trunk ports on the root bridge are designated ports.

5. Why other options are incorrect
- Root port: only exists on non-root switches and identifies the single best port toward the root; the root bridge has no root port.  
- Alternate port: is a backup (blocked) port used to prevent loops; active trunk ports on the root bridge are forwarding, not alternate/blocked.  
- Disabled port: administratively shut down; the question asks about active trunk ports, so they are not disabled.

6. Conclusion
Therefore, the correct role assigned to all active trunk switch ports on the root bridge is "designated port," matching the provided answer.

Which port role will be assigned to all active trunk switch ports on the root bridge of a Layer 2 switched domain?单项选择题

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