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Difficulty: EasySTP Port States and PortFast Benefits

A network technician notices that connecting an end-user host to an access port does not generate a Topology Change Notification (TCN) or cause MAC address table flushing on neighboring switches. Which feature configured on the access interface is responsible for suppressing these STP topology change notifications upon link state changes?

  1. PortFastAnswer
  2. B
    BPDU Guard
  3. C
    Root Guard
  4. D
    Loop Guard

Answer

PortFast suppresses Topology Change Notifications (TCNs) when edge interfaces change state, avoiding unnecessary MAC table flushing across the network.
PortFast is intended for switch ports connected to end devices. Besides bypassing the Listening and Learning states to transition immediately to Forwarding, PortFast ensures that link state changes on edge ports do not trigger Spanning Tree Topology Change Notifications (TCNs), thereby protecting the network from unnecessary MAC address table aging and flushing.

Step-by-Step Solution

1
Identify the STP behavior caused by connecting end-user devices.
Standard 802.1D STP generates a Topology Change Notification (TCN) whenever a link comes up or goes down, causing switches in the broadcast domain to shorten their MAC address table aging timer.
End-user workstations power on and off frequently, which would cause constant MAC table flushing if TCNs were transmitted.
2
Evaluate the role of PortFast.
PortFast designates a port as an edge port, immediately transitioning it to Forwarding while suppressing TCN generation upon link state changes.
Since end-user workstations do not interconnect switches, link state changes on these ports do not affect the overall Spanning Tree topology.

Key Concept

PortFast TCN Suppression and Edge Port Operation
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Topics

STP Port States and PortFast Benefits

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