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

Question

BlackTom AI · Accepted Answer

RSTP (Rapid Spanning Tree Protocol) uses a default hello time of 2 seconds, so a Cisco switch running RSTP sends BPDU frames every 2 seconds.

1) Relevant knowledge points and concepts
- BPDU (Bridge Protocol Data Unit) frames are the messages switches use to exchange spanning-tree information.
- The IEEE 802.1w RSTP standard retains the concept of a hello time that determines the regular BPDU transmission interval.
- Default Spanning Tree timer values commonly used by Cisco (and in the standard) include hello time = 2 seconds, forward delay = 15 seconds, and max age = 20 seconds.

2) Complete reasoning process step by step
- RSTP replaces traditional STP’s slower convergence mechanisms but continues to use periodic BPDUs to maintain topology information.
- The interval at which these periodic BPDUs are sent is the hello time.
- The IEEE 802.1w/RSTP default hello time is 2 seconds, and Cisco’s implementation follows this default for RSTP (and Rapid PVST+ per VLAN).

3) Intermediate steps/calculation
- No numerical calculation is required beyond recognizing the standard interval: hello time = 2 seconds → BPDU interval = 2 seconds.

4) Why the correct answer is correct
- Because the RSTP standard (802.1w) sets the default hello time to 2 seconds and Cisco’s RSTP implementation uses that same default, the switch sends out BPDUs every 2 seconds by default.

5) Why the other options are incorrect
- Every 4 seconds, every 5 seconds, and every 10 seconds are not the standard default hello intervals for RSTP; they do not match the IEEE 802.1w/Cisco default of 2 seconds. (Those larger values would slow topology detection and are not the default.)

6) Conclusion confirming the final answer
- Therefore, the correct answer is: every 2 seconds. This matches the provided authoritative answer.

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

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