350-001 · Question #145
Under which two circumstances would an RSTP bridge flush its CAM table? (Choose two.)
The correct answer is B. upon receiving a topology change notification C. when transitioning from discarding to forwarding. First, the goal of RSTP is fast re-convergence. Since ports are assumed to transition to forwarding relatively fast, simply increasing MAC address aging speed is not enough. Thus, when a topology change is detected, RSTP instructs the bridge to flush all MAC address table…
Question
Under which two circumstances would an RSTP bridge flush its CAM table? (Choose two.)
Options
- Aupon a port state change
- Bupon receiving a topology change notification
- Cwhen transitioning from discarding to forwarding
- Dwhen transitioning from forwarding to discarding
- Eonly when changing from listening to discarding
- Fwhen CAM resources have been completely used up
How the community answered
(47 responses)- A17% (8)
- B70% (33)
- D2% (1)
- E4% (2)
- F6% (3)
Explanation
First, the goal of RSTP is fast re-convergence. Since ports are assumed to transition to forwarding relatively fast, simply increasing MAC address aging speed is not enough. Thus, when a topology change is detected, RSTP instructs the bridge to flush all MAC address table entries. With Ethernet, this process results in unconstrained flooding until the moment MAC addresses are re-learned. The bridge detecting a topology change sets the TC (Topology Change) bit in all outgoing BPDUs and starts sending BPDUs with the TC bit set upstream through the root port as well. This marking lasts for TCWhile=2xHelloTime seconds and allows the detecting bridge the start the flooding process.
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