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 BPDU guard
B.
loop guard
C.
STP Root guard
D.
EtherChannel guard
Explanation:
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-treeprotocol/10588-74.html