Loop guard and UniDireclional Link Detection both protect against Layer 2 STP loops. In which
two ways does loop guard differ from UDLD in loop detection and prevention? (Choose two.)
A.
Loop guard can be used with root guard simultaneously on the same port on the same VLAN
while UDLD cannot.
B.
UDLD protects against STP failures caused by cabling problems that create one-way links.
C.
Loop guard detects and protects against duplicate packets being received and transmitted on
different ports.
D.
UDLD protects against unidirectional cabling problems on copper and fiber media.
E.
Loop guard protects against STP failures caused by problems that result in the loss of BPDUs
from a designated switch port.
Explanation:
Answers B, D, & E are all correct. However, as UDLD is only enabled on Fiber media by default I
have selected B instead of D as you will have to manually configure UDLD if you want it to work on
copper media.
The Cisco-proprietary UDLD protocol allows devices connected through fiber-optic or copper (for
example, Category 5 cabling) Ethernet cables connected to LAN ports to monitor the physical
configuration of the cables and detect when a unidirectional link exists. When a unidirectional link
is detected, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can
cause a variety of problems, including spanning tree topology loops.
UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status
of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD
performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors
and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD,
Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional
connections and the malfunctioning of other protocols.
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 thewhole 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.