If a switch port goes directly into a blocked state only when a superior BPDU is received, what mechanism must
be in use?

A.
STP root guard

B.
Loop guard

C.
STP BPDU guard

D.
Etherchannel guard

Explanation:
Brad
Answer- A
Confidence level: 100%
Remember: The phrase “only superior BPDUs” is the key to the correct answer. BPDU guard will block a port if
*ANY* BPDU is received.
BD
Root guard allows the device to participate in STP as long as the device does not try to become the root. If root
guard blocks the port, subsequent recovery is automatic. Recovery occurs as soon as the offending device
ceases to send superior BPDUs.
Source: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10588-74.html