What two features in Cisco switches help prevent Layer 2 loops? (Choose two.)
A.
UniDirectional Link Detection
B.
Hot Standby Router Protocol
C.
Virtual Router Redundancy Protocol
D.
PortFast
E.
root guard
F.
loop guard
Explanation:
Loop Guard versus UDLD
Loop guard and Unidirectional Link Detection (UDLD) functionality overlap, partly in the sense that
both protect against STP failures caused by unidirectional links. However, these two features differ
in functionality and how they approach the problem.Based on the various design considerations, you can choose either UDLD or the loop guard feature.
In regards to STP, the most noticeable difference between the two features is the absence of
protection in UDLD against STP failures caused by problems in software. As a result, the designated
switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare
than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of
unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel
should remain functional with the links that remain. In such a failure, the loop guard puts it into
loop-inconsistent state in order to block the whole channel.
Additionally, loop guard does not work on shared links or in situations where the link has been
unidirectional since the link-up. In the last case, the port never receives BPDU and becomes
designated. Because this behaviour could be normal, this particular case is not covered by loop
guard. UDLD provides protection against such a scenario.