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Difficulty: HardSTP Port States and PortFast Benefits

A network administrator is troubleshooting an issue where end-user workstations experience initial network connection timeouts and fail to obtain IPv4 addresses via DHCP upon booting up. The administrator decides to configure Cisco STP PortFast on the access-mode switch interfaces connected to these workstations. Which TWO operational benefits or behaviors occur as a result of enabling PortFast on these access ports? (Select TWO.)

  1. The interface bypasses the standard 802.1D Listening and Learning states, transitioning immediately to the Forwarding state upon link activation.Answer
  2. Link state changes on the interface do not trigger Topology Change Notifications (TCNs) to be flooded across the Spanning Tree domain.Answer
  3. C
    Spanning Tree Protocol is completely disabled on the interface, preventing outbound Bridge Protocol Data Units (BPDUs) from being transmitted.
  4. D
    The interface is automatically protected against Layer 2 switching loops if an unmanaged switch or bridge is mistakenly connected to the port.

Answer

Enabling PortFast on access interfaces allows ports to immediately transition from the Blocking state to the Forwarding state by bypassing the standard Listening and Learning delay, while also suppressing the generation of Topology Change Notifications (TCNs) during link UP/DOWN events on edge ports.
The correct options identify that PortFast allows an access port to transition directly from Blocking to Forwarding (bypassing the 15-second Listening and 15-second Learning timer delay) and that edge interface transitions on PortFast ports do not trigger Topology Change Notifications (TCNs) to the STP root bridge.

Step-by-Step Solution

1
Analyze the impact of standard 802.1D STP convergence on DHCP client requests.
Standard IEEE 802.1D STP requires ports to spend 15 seconds in the Listening state and 15 seconds in the Learning state (30 seconds total) before reaching Forwarding.
This 30-second convergence delay causes DHCP client software on workstations to time out before the port can forward traffic.
2
Determine the primary operational state behavior of Cisco PortFast.
PortFast moves an edge interface immediately into the Forwarding state upon link UP, bypassing Listening and Learning.
End-user devices do not create Layer 2 loops, making the 30-second STP listening/learning check unnecessary for edge devices.
3
Evaluate the secondary operational trait regarding Topology Change Notifications (TCNs).
PortFast suppresses TCN generation for edge port link status changes.
Frequent rebooting or power cycles of client PCs would otherwise cause constant network-wide MAC address table aging if TCNs were flooded.

Key Concept

STP PortFast Operational Behavior and TCN Suppression
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Topics

STP Port States and PortFast Benefits

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