Cisco CCNA Exam: Interface State with BPDU Guard and PortFast Enable

BPDU Guard and PortFast: Interface State

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A BPDU guard is configured on an interface that has PortFast Enable. Which state does the interface enter when it receives a BPDU?



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A. B. C. D.


When PortFast is enabled on a Cisco switch interface, it bypasses the Spanning Tree Protocol (STP) process and immediately transitions the interface to forwarding state, without waiting for the normal STP convergence time. This allows end devices such as workstations or servers to connect and get network connectivity quickly, rather than waiting for the switch to complete the STP process.

However, in some cases, a BPDU (Bridge Protocol Data Unit) packet might still be received on a PortFast-enabled interface. BPDU packets are used by STP to communicate with other switches and prevent loops in the network. If a BPDU packet is received on a PortFast-enabled interface, it could indicate that another switch has been connected to the interface, which could cause a loop in the network if the interface stays in forwarding state.

To prevent this, Cisco switches can be configured with BPDU guard, which will automatically put the interface into an error-disabled state if a BPDU packet is received on the interface. This protects the network from loops and other STP-related issues.

Therefore, the answer to the question is D. Errdisable. If a BPDU is received on an interface with PortFast and BPDU guard enabled, the interface will be put into the errdisable state, which disables the interface and puts it into an error condition. The administrator must then manually re-enable the interface after addressing the issue that caused the error. This provides an additional layer of protection for the network and helps prevent misconfigurations or malicious activities.