A port in a redundant topology is currently in the blocking state and is not receiving BPDUs. To
ensure that this port does not erroneously transition to the forwarding state, which command
should be configured?

A.
Switch(config)#spanning-tree loopguard default

B.
Switch(config-if)#spanning-tree bdpufilter

C.
Switch(config)#udld aggressive

D.
Switch(config-if)#spanning-tree bpduguard

Explanation:
The STP loop guard feature provides additional protection against Layer 2
forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant
topology erroneously transitions to the forwarding state. This usually happens because one of the
ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives
STP BPDUs. In its operation, STP relies on continuous reception or transmission of BPDUs based
on the port role. The designated port transmits BPDUs, and the non-designated port receives
BPDUs. When one of the ports in a physically redundant topology no longer receives BPDUs, the
STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or
backup port becomes designated and moves to a forwarding state. This situation creates a loop.
The loop guard feature makes additional checks. If BPDUs are not received on a non-designated
port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state,
instead of the listening / learning / forwarding state. Without the loop guard feature, the port
assumes the designated port role. The port moves to the STP forwarding state and creates a loop.
Reference:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml#l
oop_guard