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352-001 · Question #374

Which option describes an advantage of UDLD?

The correct answer is B. It prevents switch loops by detecting one-way communications on the physical port. UDLD detects unidirectional link conditions - where traffic flows in only one direction - and disables the affected port to prevent forwarding loops that STP would not catch.

Layer 2 Control Plane

Question

Which option describes an advantage of UDLD?

Options

  • AIt makes the port go immediately into the forwarding state after being connected.
  • BIt prevents switch loops by detecting one-way communications on the physical port.
  • CIt prevents switched traffic from traversing suboptimal paths on the network.
  • DIt prevents switch loops caused by uni-directional point-to-point link condition on Rapid PVST+
  • EIt does not generate a spanning-tree topology change upon connecting and disconnecting a
  • FIt allows small, unmanaged switches to be plugged into ports of access switches without the risk

How the community answered

(57 responses)
  • A
    4% (2)
  • B
    86% (49)
  • C
    2% (1)
  • D
    2% (1)
  • E
    7% (4)

Why each option

UDLD detects unidirectional link conditions - where traffic flows in only one direction - and disables the affected port to prevent forwarding loops that STP would not catch.

AIt makes the port go immediately into the forwarding state after being connected.

Bypassing the listening and learning states to immediately enter forwarding is the function of PortFast, not UDLD.

BIt prevents switch loops by detecting one-way communications on the physical port.Correct

UDLD works by sending protocol packets and expecting them to be echoed back by the neighbor. If a fiber strand or copper pair fails in one direction, BPDUs may never reach the peer while the port remains physically up, creating a loop STP cannot detect. UDLD identifies this one-way communication condition and err-disables the port, preventing the loop.

CIt prevents switched traffic from traversing suboptimal paths on the network.

Preventing traffic from taking suboptimal paths is the function of Spanning Tree Protocol and its variants, not UDLD.

DIt prevents switch loops caused by uni-directional point-to-point link condition on Rapid PVST+

UDLD is not specific to Rapid PVST+ and does not itself prevent loops caused by STP configuration - it addresses physical unidirectional link failures that STP cannot detect.

EIt does not generate a spanning-tree topology change upon connecting and disconnecting a

Suppressing topology change notifications on link events is the behavior of PortFast, not UDLD.

FIt allows small, unmanaged switches to be plugged into ports of access switches without the risk

Preventing unauthorized switch attachment to access ports is the function of BPDU Guard, not UDLD.

Concept tested: UDLD detection of unidirectional physical link failures

Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/12-2SX/configuration/guide/book/udld.html

Topics

#UDLD#one-way link detection#Layer 2 loop prevention#port state

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