Bridge Protocol Data Units (BPDUs) How frequently are BPDUs (Bridge Protocol Data Units) sent out by a Layer 2 device in a Spanning Tree Protocol (STP) enabled network?
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ANever
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BEvery 2 seconds
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CEvery 10 minutes
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DEvery 30 seconds
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EEvery 60 seconds
Answer
Correct Answer: Every 2 seconds
Explanation
Introduction / Context:BPDUs (Bridge Protocol Data Units) are messages exchanged by switches in a network running the Spanning Tree Protocol (STP). They are crucial in preventing loops and maintaining a loop-free topology. Their transmission frequency is standardized to ensure stable convergence.
Given Data / Assumptions:
- We are considering IEEE 802.1D STP standard.
- Default configuration is assumed.
- BPDU timers are unchanged (Hello Time = 2 seconds).
Concept / Approach:
In IEEE 802.1D, switches send BPDUs by default every 2 seconds. These Hello BPDUs allow switches to share spanning tree information and elect the root bridge.
Step-by-Step Solution:
Recall the default Hello Time is 2 seconds.Verify that changes to Hello Time can alter this interval, but by default it is 2 seconds.Therefore, the correct answer is Every 2 seconds.Verification / Alternative check:
Cisco networking documentation and IEEE 802.1D confirm the Hello BPDU interval of 2 seconds as the default setting.
Why Other Options Are Wrong:
- Never: incorrect; BPDUs are the foundation of STP.
- 10 minutes or 30 seconds: too infrequent; would cause instability.
- 60 seconds: not a standard default timer.
Common Pitfalls:
- Confusing BPDU interval with Max Age timer (20 seconds).
- Assuming BPDUs are sent only by the root bridge; all switches generate them.
Final Answer:
Every 2 seconds