Which two features can prevent the same issues in the future?

Your network is suffering from regular outages. After troubleshooting, you learn that the transmit
lead of a fiber uplink was damaged. Which two features can prevent the same issues in the
future? (Choose two.)

A.
Root guard

B.
Loop guard

C.
BPDU guard

D.
UDLD

E.
BPDU skew detection

Explanation:
STP Loop Guard
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.
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. This table describes loop guard and UDLD
functionality:

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 behavior could be normal, this particular case is not covered by loop
guard. UDLD provides protection against such a scenario.
As described, the highest level of protection is provided when you enable UDLD and loop guard.

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