350-001 Question #114: Real Exam Question with Answer & Explanation

Question

Options

• ALoop guard can be used with root guard simultaneously on the same port on the same VLAN while
• BUDLD protects against STP failures caused by cabling problems that create one-way links.
• CLoop guard detects and protects against duplicate packets being received and transmitted on
• DUDLD protects against unidirectional cabling problems on copper and fiber media.
• ELoop guard protects against STP failures caused by problems that result in the loss of BPDUs

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 the 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 UDLD provides protection against such a scenario.

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