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Difficulty: MediumSTP Port States and PortFast Benefits

A network administrator is deploying switch configurations for end-user workstation ports in an enterprise network. Which two statements correctly describe the operational benefits and state transition changes when Cisco STP PortFast is enabled on an access interface?

  1. The switch port bypasses the 802.1D Listening and Learning states, transitioning immediately from Blocking to Forwarding upon link bring-up.Answer
  2. Link state changes on the interface do not trigger Topology Change Notifications (TCNs) sent across the Spanning Tree domain.Answer
  3. C
    The switch port automatically filters all incoming Bridge Protocol Data Units (BPDUs) to prevent rogue switches from altering the STP topology.
  4. D
    Enabling PortFast on inter-switch trunk links provides rapid convergence without risking Layer 2 forwarding loops.

Answer

PortFast allows an access interface to bypass the Listening and Learning states to transition immediately to Forwarding, and it prevents interface status changes from generating Topology Change Notifications (TCNs) across the network.
PortFast provides two major operational benefits: it allows edge access ports to skip the 15-second Listening and 15-second Learning states to enter Forwarding instantly, and it prevents link up/down events on host ports from generating Topology Change Notifications (TCNs) that unnecessarily flush MAC tables network-wide.

Step-by-Step Solution

1
Analyze the state transition behavior of PortFast.
Standard 802.1D STP takes 30 seconds (15s Listening + 15s Learning) before forwarding frames. PortFast bypasses these intermediary states to put the link into Forwarding immediately upon physical link detection.
End-user hosts need instantaneous network connectivity to complete initial protocol exchanges such as DHCP IP address assignment.
2
Analyze the impact of PortFast on STP topology change notifications (TCNs).
Normal STP topology changes trigger TCN BPDUs, instructing switches to reduce MAC table aging times. PortFast suppresses TCN generation for host connect/disconnect events.
Host link state toggles do not alter the physical path between switches, so flushing MAC address tables across the campus fabric is unnecessary.
3
Evaluate distractor statements regarding BPDU handling and trunk port usage.
PortFast alone does not filter BPDUs, nor is it safe for switch-to-switch interconnects.
Receiving a BPDU on a standard PortFast port causes it to lose PortFast status unless BPDU Guard is active, and enabling PortFast on trunks risks creating Layer 2 loops.

Key Concept

STP PortFast operational benefits and state transitions
Estimated Time:1m 15s
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Topics

STP Port States and PortFast Benefits

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