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 nondesignated 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 nondesignated 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#loop_guard