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352-001 · Question #787

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 devices connect or disconnect, preventing unnecessary MAC address table flushes across the network.

Layer 2 Control Plane

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

(58 responses)
  • A
    90% (52)
  • B
    2% (1)
  • D
    3% (2)
  • E
    5% (3)

Why each option

PortFast suppresses Topology Change Notifications (TCNs) when end devices 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

When PortFast is enabled on an access port, STP does not generate a TCN when a station connects or disconnects from that port. Without PortFast, every link state change triggers a TCN that causes all switches to flush their MAC tables, increasing flooding. PortFast eliminates this churn on ports connected to end hosts.

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

PortFast does not disable STP on the port - the port still participates in STP and can be blocked if a superior BPDU is received; it only skips the listening and learning states on initial linkup.

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

Plugging unmanaged switches into PortFast-enabled ports is explicitly discouraged because it bypasses loop-detection delays and can cause bridging loops; PortFast is designed for end-host ports only.

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 switch loops caused by unidirectional point-to-point link conditions is the role of Loop Guard or UDLD, not PortFast.

FPortFast prevents switched traffic from traversing suboptimal paths on the network

Preventing switched traffic from traversing suboptimal paths is the core purpose of the Spanning Tree Protocol itself, not a specific PortFast benefit.

Concept tested: PortFast TCN suppression behavior on access ports

Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-19.html

Topics

#PortFast#STP#topology change notification#access port

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