An administrator is troubleshooting a switch and wants to verify if it is a root bridge. What command can be used to do this?

Question

BlackTom AI · Accepted Answer

Spanning Tree Protocol elects the root bridge using the lowest Bridge ID (priority + system MAC). The IOS command that reports the current root status and Bridge ID for each VLAN is show spanning-tree.

1) Relevant concepts
- Bridge ID = (priority field) + (system MAC). Default priority is typically 32768; the system ID extension incorporates the VLAN when applicable.
- STP election: the switch with the lowest Bridge ID becomes the root. If priorities tie, the lowest MAC address wins.
- Operational state vs. configuration: you must inspect runtime STP information to know which device is the current root.

2) Step-by-step reasoning to verify root bridge
- Execute show spanning-tree on the switch.
- Read the output for each VLAN: locate the "Root ID" and "Bridge ID" fields.
- Compare the two IDs: if Bridge ID equals Root ID for a VLAN, this switch is the root bridge for that VLAN. If they differ, the Root ID identifies another switch as root.

3) Example and intermediate steps (interpretation)
- Example output excerpt:
  Root ID    Priority    32768
             Address     0011.2233.4455
  Bridge ID  Priority    32768
             Address     0011.2233.4455
- Intermediate check: Bridge ID (32768 + 0011.2233.4455) equals Root ID (32768 + 0011.2233.4455), so this switch is root.
- If Bridge ID were 32768.00aa.bbccdd while Root ID is 32768.0011.2233.4455, they differ → not root.

4) Why show spanning-tree is correct
- show spanning-tree is the operational troubleshooting command that explicitly displays Root ID and Bridge ID; it directly answers whether the local switch is the root for each VLAN.

5) Why other options are incorrect
- show running-config: shows saved runtime configuration (STP settings may appear) but does not directly indicate which device is currently the root in the topology.
- show startup-config: shows the saved configuration that will be used at boot; it does not reflect current root status.
- show vlan: displays VLAN membership information, not STP root/Bridge ID details.

6) Conclusion
Comparing Root ID and Bridge ID from the show spanning-tree output confirms root status, so the correct answer is: show spanning-tree. This matches the provided answer.

An administrator is troubleshooting a switch and wants to verify if it is a root bridge. What command can be used to do this?单项选择题

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