Refer to the exhibit. Look at the command output. What can you use to prevent this behavior?

A.
udld

B.
spanning-tree loopguard

C.
VTP mode transparent

D.
switchport mode desirable

Explanation:
UDLD Overview
The Cisco-proprietary Unidirectional Link Detection (UDLD) protocol allows ports that are
connected through fiber optics or copper (for example, Category 5 cabling) Ethernet cables to
monitor the physical configuration of the cables and detect when a unidirectional link exists. When
the switch detects a unidirectional link, 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.
A unidirectional link occurs whenever traffic transmitted by the local device over a link is received
by the neighbor but traffic transmitted from the neighbor is not received by the local device. If one
of the fiber strands in a pair is disconnected, as long as autonegotiation is active, the link does not
stay up. In this case, the logical link is undetermined, and UDLD does not take any action. If both
fibers are working normally at Layer 1, then UDLD at Layer 2 determines whether those fibers are
connected correctly and whether traffic is flowing bidirectionally between the correct neighbors.
This check cannot be performed by autonegotiation, because
autonegotiation operates at Layer 1.
A Cisco Nexus 5000 Series switch periodically transmits UDLD frames to neighbor devices on
LAN ports with UDLD enabled. If the frames are echoed back within a specific time frame and they
lack a specific acknowledgment (echo), the link is flagged as unidirectional and the LAN port is
shut down. Devices on both ends of the link must support UDLD in order for the protocol to
successfully identify and disable unidirectional links.
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

http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli_rel_4
_0_1a/BasicEthernet.html#wp1267151
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml