What is the value used to determine which port on a non-root bridge will become a root port in a STP network?

Question

BlackTom AI · Accepted Answer

The path cost determines which port on a non-root bridge becomes the root port.

1. Relevant knowledge points and concepts
- Spanning Tree Protocol (STP) elects a single root bridge and then every other (non-root) switch selects exactly one root port: the port providing the best path to the root.
- “Path cost” is the cumulative metric representing the cost from a switch, via a given port, to the root bridge. Link costs are assigned based on link speed (higher-speed links have lower cost). STP chooses the path with the lowest total path cost.
- Tie-breakers (when path costs are equal) include the lowest sending bridge ID (bridge priority + MAC) and then the lowest port ID.

2. Step-by-step reasoning process
- Each switch receives BPDUs from neighbors advertising that neighbor’s root path cost (the cost from that neighbor to the root).
- For each local port, the switch computes the total path cost to the root as: advertised root path cost (from the BPDU) + cost of the local outbound link (the port’s link cost).
- The switch compares the total path costs computed for each port and selects the port with the smallest total as the root port.
- If two ports produce identical total path costs, the switch applies tie-breakers: compare the sending bridge IDs in the BPDUs; if still tied, compare port IDs.

3. Example calculation (intermediate steps)
- Port A receives a BPDU advertising root path cost 10; Port A’s link cost = 4. Total via A = 10 + 4 = 14.
- Port B receives a BPDU advertising root path cost 8; Port B’s link cost = 5. Total via B = 8 + 5 = 13.
- Compare totals: 13 (via B) < 14 (via A) → Port B becomes the root port.

4. Why the correct answer is correct
- STP explicitly uses path cost as the primary metric for selecting the root port on non-root bridges. The lowest cumulative path cost identifies the best route to the root.

5. Why the other options are incorrect
- Highest or lowest MAC address of ports: MAC addresses are used in tie-breakers for bridge or port IDs, but not as the primary criterion for selecting the root port.
- VTP revision number: VTP is a separate VLAN management protocol and its revision number is unrelated to STP root port selection.

6. Conclusion
- The correct value used to determine which port on a non-root bridge becomes the root port is the path cost, which matches the provided answer.

What is the value used to determine which port on a non-root bridge will become a root port in a STP network?单项选择题

类似问题

By default, how often does a Cisco switch that is using RSTP send out BPDU frames?

Which three port states are used by Rapid PVST+? (Choose three.)

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

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

In which two port states does a switch learn MAC addresses and process BPDUs in a PVST network? (Choose two.)

Which RSTP ports are connected to end devices?

What is the purpose of STP?

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

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单