Which two statements about root guard are true? (Choose two.)
A. It supports the errdisable-timeout command.
B. It allows unknown devices to participate in STP.
C. It requires the administrator to manually re-enable ports that have been blocked.
D. It automatically error-disables a port when PortFast is enabled.
E. It automatically re-enables blocked ports.
F. It blocks BPDUs from unknown devices.
The document from cisco states the opposite: “After device D ceases to send superior BPDUs, the port is unblocked again. Via STP, the port goes from the listening state to the learning state, and eventually transitions to the forwarding state. Recovery is automatic; no human intervention is necessary.“, so E is correct and C is not. B is correct as well, since it does not prevent unknown devices from partecipating in stp, but just prevents that they become root
correct answer is A,C