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400-007 · Question #66

Which design benefit of PortF ast is true?

The correct answer is A. PortFast does not generate a spanning tree topology change hen a station on a port is connected. PortFast suppresses Topology Change Notifications (TCNs) when end stations connect or disconnect, preventing unnecessary MAC address table flushes across the network.

Designing Network Infrastructure

Question

Which design benefit of PortF ast is true?

Options

  • APortFast does not generate a spanning tree topology change hen a station on a port is connected
  • BPortFast disables spanning tree on the port, which puts the port into the forwarding state
  • CPortFast allows small, unmanaged switches to be plugged into ports of access switches without
  • DPortFast detects one-way communications on the physical port, which prevents switch loops
  • EPortFast prevents switch loops that are caused by a unidirectional point to point link condition on
  • FPortFast prevents switched traffic from traversing suboptimal paths on the network

How the community answered

(45 responses)
  • A
    93% (42)
  • D
    4% (2)
  • F
    2% (1)

Why each option

PortFast suppresses Topology Change Notifications (TCNs) when end stations connect or disconnect, preventing unnecessary MAC address table flushes across the network.

APortFast does not generate a spanning tree topology change hen a station on a port is connectedCorrect

Normally, when a port transitions to forwarding, STP generates a Topology Change Notification that causes bridges to flush their MAC tables, increasing flooding. PortFast suppresses these TCNs on access ports because end stations do not create topology changes that affect the spanning tree, reducing unnecessary network disruption.

BPortFast disables spanning tree on the port, which puts the port into the forwarding state

PortFast does not disable STP on the port - STP BPDUs are still processed, and receiving a BPDU on a PortFast port causes it to lose PortFast behavior; the port simply skips the listening and learning states.

CPortFast allows small, unmanaged switches to be plugged into ports of access switches without

Allowing unmanaged switches into PortFast-enabled ports is actually dangerous, not a benefit, because a switch can introduce a loop; BPDU Guard is the companion feature used to protect against this.

DPortFast detects one-way communications on the physical port, which prevents switch loops

Detecting one-way communications on a physical link is the function of UDLD (UniDirectional Link Detection), not PortFast.

EPortFast prevents switch loops that are caused by a unidirectional point to point link condition on

Preventing loops caused by unidirectional point-to-point link conditions is the purpose of UDLD, not PortFast.

FPortFast prevents switched traffic from traversing suboptimal paths on the network

Preventing suboptimal path traversal is a function of STP root bridge placement or loop guard, not PortFast.

Concept tested: PortFast TCN suppression behavior in STP

Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html

Topics

#PortFast#Spanning Tree Protocol#topology change#access layer

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