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單項選擇題

Switch R1 has been configured with the root guard feature. 
What statement is true if the spanning  tree enhancement Root Guard is enabled？ （）

A. If BPDUs are not received on a non-designated port， the port is moved into the STP loop- inconsistent blocked state
B. IF BPDUs are received on a PortFast enabled port， the port is disabled.
C. If superior BPDUs are received on a designated port， the interface is placed into the root- inconsistent blocked state.
D. If inferior BPDUs are received on a root port， all blocked ports become alternate paths to the  root bride.

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